Display device and electronic appliance

ABSTRACT

A display device with low manufacturing cost, a display device with low power consumption, a display device capable of being formed over a large substrate, a display device with a high aperture ratio of a pixel, and a display device with high reliability are provided. The display device includes a transistor electrically connected to a light-transmitting pixel electrode and a capacitor. The transistor includes a gate electrode, a gate insulating film, and a first multilayer film including an oxide semiconductor layer. The capacitor includes the pixel electrode and a second multilayer film overlapping with the pixel electrode, positioned at a predetermined distance from the pixel electrode, and having the same layer structure as the first multilayer film. A channel formation region of the transistor is at least one layer, which is not in contact with the gate insulating film, of the first multilayer film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed in this specification and the like relates to adisplay device and an electronic appliance.

2. Description of the Related Art

In recent years, flat panel displays such as liquid crystal displays andorganic EL displays have been widely used. In each of pixels provided inthe row direction and the column direction in a display device such as aflat panel display, a transistor serving as a switching element, adisplay element electrically connected to the transistor, and acapacitor connected to the display element are provided.

As a silicon semiconductor film which is used for a transistor, eitheran amorphous silicon film or a polycrystalline silicon film is useddepending on the purpose. For example, in the case where a displaydevice is manufactured over a large substrate, a transistor using anamorphous silicon film, which can be formed using the establishedtechnique for forming a film over a large substrate, is preferable.Manufacturing the display device over the large substrate can reducemanufacturing costs of the display device. However, since a transistorusing the amorphous silicon film has low field-effect mobility, the areaof the transistor needs to be increased for sufficient on-state current.The aperture ratio of a pixel decreases as the area of the transistorincreases, which results in an increase in power consumption of thedisplay device.

In contrast, a transistor using a polycrystalline silicon film has highfield-effect mobility; thus, sufficient on-state current can be obtainedeven when the transistor has a small area. Thus, the aperture ratio of apixel can be increased, which results in a reduction in powerconsumption of a display device. However, the polycrystalline siliconfilm is formed by performing high-temperature heat treatment or laserlight treatment on an amorphous silicon film, and thus is difficult tobe formed over a large substrate. Since the display device is difficultto be manufactured over the large substrate, manufacturing costs of thedisplay device is increased.

Oxides having semiconductor characteristics (also referred to as oxidesemiconductors) are semiconductor materials that can be used forsemiconductor films in transistors. For example, a technique by which atransistor is manufactured using zinc oxide or an In—Ga—Zn-based oxideis disclosed (see Patent Documents 1 and 2).

An oxide semiconductor film can be formed by a sputtering method, andthus is suitable for manufacturing a display device over a largesubstrate. Manufacturing the display device over the large substrate canreduce the manufacturing costs of the display device. A transistor usingthe oxide semiconductor film has high field-effect mobility; thus,sufficient on-state current can be obtained even when the transistor hasa small area. Thus, the aperture ratio of a pixel can be increased,which results in a reduction in the power consumption of the displaydevice. In addition, there is an advantage that capital investment canbe reduced because part of production equipment for a transistorincluding an amorphous silicon film can be retrofitted and utilized.

As the resolution of the display device increases, the area occupied bya wiring, an electrode, and the like increases; thus, the aperture ratioof a pixel is reduced, which results in an increase in the powerconsumption of the display device. For example, in the case where thewidth of a wiring is reduced in order to increase the aperture ratio ofa pixel, the operation of the display device is delayed, which resultsin deterioration of the display quality of the display device in somecases. Also in the case where the size of a capacitor is reduced inorder to increase the aperture ratio of a pixel, deterioration of thedisplay quality of the display device is caused in some cases.

An oxide semiconductor film is known to transmit visible light becauseof its energy gap as wide as approximately 3 eV to 4 eV. Patent Document3 discloses that in a display device, a channel layer of a transistorand one capacitor electrode used for a capacitor are formed on the samesurface as a light-transmitting oxide semiconductor film. The othercapacitor electrode used for the capacitor is formed of alight-transmitting pixel electrode; thus, the capacitor as a whole canbe transparent.

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055-   [Patent Document 3] U.S. Pat. No. 8,102,476

SUMMARY OF THE INVENTION

An object is to provide a display device with low manufacturing costs.Another object is to provide a display device with low powerconsumption. Another object is to provide a display device which can bemanufactured over a large substrate. Another object is to provide adisplay device with a high aperture ratio of a pixel. Another object isto provide a display device with high reliability.

One embodiment of the present invention is a display device whichincludes a transistor electrically connected to a light-transmittingpixel electrode and a capacitor. The transistor includes a gateelectrode, a gate insulating film over the gate electrode, and a firstmultilayer film including an oxide semiconductor layer which is over thegate insulating film. The capacitor includes a pixel electrode and asecond multilayer film which overlaps with the pixel electrode, ispositioned at a predetermined distance from the pixel electrode, and hasthe same layer structure as the first multilayer film. A channelformation region of the transistor is at least one layer, which is notin contact with the gate insulating film, of the first multilayer film.

In order to provide stable electrical characteristics for a transistorwhose channel is formed in a multilayer film including an oxidesemiconductor layer, it is effective to reduce the concentration ofimpurities in a layer in which a channel is formed in some cases. Forexample, in some cases, silicon in an oxide semiconductor forms impuritystates. Further, in some cases, the impurity states serve as traps todeteriorate the electrical characteristics of the transistor. Note thatsince an insulating film containing silicon, such as a silicon oxidefilm, a silicon oxynitride film, a silicon nitride film, or a siliconnitride oxide film is used for the gate insulating film of thetransistor in many cases, it is preferable that a layer, in which thechannel is formed, of the multilayer film including the oxidesemiconductor layer be not in contact with the gate insulating film.

In the case where a channel is formed at an interface between the gateinsulating film and the multilayer film including the oxidesemiconductor layer, interface scattering occurs at the interface andthe field-effect mobility of the transistor is decreased. In view of theabove, the channel of the transistor is preferably formed in a layer,which is not in contact with the gate insulating film, of the multilayerfilm including the oxide semiconductor layer.

Accordingly, the channel of the transistor is formed in the layer, whichis not in contact with the gate insulating film, of the multilayer filmincluding the oxide semiconductor layer, whereby the transistor can havestable electrical characteristics and high field-effect mobility. Whenthe transistor is used as a switching element of a display device, thedisplay device can have high reliability because the transistor has thestable electrical characteristics. Further, since the transistor canobtain sufficient on-state current even when it has a small area, theaperture ratio of a pixel can be increased, which results in a reductionin the power consumption of the display device.

For example, the multilayer film including the oxide semiconductor layerpreferably has the structure described below in order that the channelformation region of the transistor is located away from the gateinsulating film.

The multilayer film including the oxide semiconductor layer includes atleast an oxide semiconductor layer (referred to as second oxide layerfor convenience) and a first oxide layer between the second oxide layerand the gate insulating film. The first oxide layer is an oxide filmwhich is formed of one or more kinds of elements other than oxygen,which form the second oxide layer and has electron affinity which islower than that of the second oxide layer by 0.2 eV or more. At thistime, when an electric field is applied to the gate electrode, a channelis formed in the second oxide layer having a higher electron affinity inthe multilayer film including the oxide semiconductor layer. In otherwords, forming the first oxide layer between the second oxide layer andthe gate insulating film enables the channel of the transistor to beformed in the layer (here, the second oxide layer) which is not incontact with the gate insulating film. Further, since the first oxidelayer is formed of one or more kinds of elements other than oxygen,which form the second oxide layer, interface scattering is less likelyto occur at an interface between the first oxide layer and the secondoxide layer. Thus, carriers are not inhibited from moving at theinterface, which results in an increase in the field-effect mobility ofthe transistor.

The first oxide layer may be, for example, an oxide film containingaluminum, silicon, titanium, gallium, germanium, yttrium, zirconium,tin, lanthanum, cerium, or hafnium at a concentration higher than thatin the second oxide layer. Specifically, as the first oxide layer, anoxide film containing the above element at a concentration 1.5 times ormore, preferably twice or more, more preferably 3 times or more that inthe second oxide layer is used. The above element is strongly bonded tooxygen, and thus has a function of suppressing generation of oxygenvacancies in the oxide film. In other words, the first oxide layer is anoxide film in which oxygen vacancies are less likely to be generatedthan in the second oxide layer.

Alternatively, in the case where the second oxide layer is anIn—Ga—Zn-based oxide having an atomic ratio of In:Ga:Zn=x₂:y₂:z₂ and thefirst oxide layer is also an In—Ga—Zn-based oxide having an atomic ratioof In:Ga:Zn=x₁:y₁:z₁, the first oxide layer and the second oxide layerwhich satisfy the following conditions are selected: y₁/x₁ is largerthan y₂/x₂, preferably y₁/x₁ is 1.5 times or more as large as y₂/x₂,more preferably y₁/x₁ is twice or more as large as y₂/x₂, furtherpreferably y₁/x₁ is 3 times or more as large as y₂/x₂.

The multilayer film including the oxide semiconductor layer may includea third oxide layer on the side which is not in contact with the gateinsulating film, which is in contact with the second oxide layer, isformed of one or more kinds of elements other than oxygen, which formthe second oxide layer, and has a lower electron affinity than thesecond oxide layer by 0.2 eV or more. At this time, a channel is notformed in the third oxide layer even when an electric field is appliedto the gate electrode. Further, since the third oxide layer is formed ofone or more kinds of elements other than oxygen, which form the secondoxide layer, an interface state is less likely to be formed at aninterface between the second oxide layer and the third oxide layer. Whenthe interface has an interface state, a second transistor in which theinterface serves as a channel formation region and which has differentthreshold voltage is formed and the apparent threshold voltage of thetransistor varies in some cases. Thus, providing the third oxide layermakes it possible to reduce variation in the electrical characteristicsof the transistor, such as threshold voltage.

The third oxide layer may be, for example, an oxide film containingaluminum, silicon, titanium, gallium, germanium, yttrium, zirconium,tin, lanthanum, cerium, or hafnium at a concentration higher than thatin the second oxide layer. Specifically, as the third oxide layer, anoxide film containing the above element at a concentration 1.5 times ormore, preferably twice or more, more preferably 3 times or more that inthe second oxide layer is used. The above element is strongly bonded tooxygen, and thus has a function of suppressing generation of oxygenvacancies in the oxide film. In other words, the third oxide layer is anoxide film in which oxygen vacancies are less likely to be generatedthan in the second oxide layer.

Alternatively, in the case where the second oxide layer is anIn—Ga—Zn-based oxide film having an atomic ratio of In:Ga:Zn=x₂:y₂:z₂and the third oxide layer is also an In—Ga—Zn-based oxide film having anatomic ratio of In:Ga:Zn=x₃:y₃:z₃, the second oxide layer and the thirdoxide layer which satisfy the following conditions are selected: y₃/x₃is larger than y₂/x₂, preferably y₃/x₃ is 1.5 times or more as large asy₂/x₂, more preferably y₃/x₃ is twice or more as large as y₂/x₂, furtherpreferably y₃/x₃ is 3 times or more as large as y₂/x₂.

In the display device according to one embodiment of the presentinvention, the capacitor includes the light-transmitting pixel electrodeand the second multilayer film which has the same layer structure as themultilayer film including the oxide semiconductor layer (forconvenience, referred to as first multilayer film) and has alight-transmitting property. The second multilayer film has alight-transmitting property, so that the capacitor can have alight-transmitting property. The use of the light-transmitting capacitorenables the aperture ratio of a pixel to be increased, which results ina reduction in the power consumption of the display device.

Note that forming the second multilayer film in the same process as thefirst multilayer film can reduce manufacturing steps of the displaydevice. The reduction in the manufacturing steps of the display devicecan result in a reduction in the manufacturing costs of the displaydevice.

The second multilayer film functions as part of the electrode becausecarriers are induced in the layer formed in the same process as at leastthe second oxide layer by an electric field applied from the pixelelectrode. Further, the layer formed in the same process as the firstoxide layer and the layer formed in the same process as the third oxidelayer each have a sufficiently high carrier density as compared to aninsulating film such as a gate insulating film; thus, these layers alsofunction as part of the electrode.

According to one embodiment of the present invention, the use of themultilayer film including the oxide semiconductor layer for thetransistor and the capacitor enables a display device to be manufacturedover a large substrate, so that the display device can have lowmanufacturing costs. In addition, since the multilayer film used for thecapacitor has a light-transmitting property, the aperture ratio of apixel can be increased, so that the display device can have low powerconsumption. Moreover, since the channel is formed in the layer, whichis not in contact with the gate insulating film, of the multilayer filmused for the transistor, the transistor can have stable electricalcharacteristics, so that the display device can have high reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a display device of one embodiment of the presentinvention and FIGS. 1B and 1C are circuit diagrams each illustrating apixel.

FIG. 2 is a top view illustrating a pixel of a display device of oneembodiment of the present invention.

FIGS. 3A to 3C are cross-sectional views illustrating a display deviceof one embodiment of the present invention.

FIGS. 4A and 4B are cross-sectional views illustrating a method formanufacturing a display device of one embodiment of the presentinvention.

FIGS. 5A and 5B are cross-sectional views illustrating a method formanufacturing a display device of one embodiment of the presentinvention,

FIG. 6 is a top view illustrating a pixel of a display device of oneembodiment of the present invention.

FIGS. 7A to 7C are cross-sectional views illustrating a display deviceof one embodiment of the present invention.

FIG. 8 is a top view illustrating a pixel of a display device of oneembodiment of the present invention.

FIGS. 9A to 9C are cross-sectional views illustrating a display deviceof one embodiment of the present invention.

FIG. 10 is a top view illustrating a pixel of a display device of oneembodiment of the present invention.

FIGS. 11A to 11C are cross-sectional views illustrating a display deviceof one embodiment of the present invention.

FIGS. 12A to 12C are top views each illustrating a display device of oneembodiment of the present invention.

FIGS. 13A and 13B are cross-sectional views each illustrating a displaydevice of one embodiment of the present invention.

FIGS. 14A to 14C are cross-sectional views and a top view eachillustrating a display device of one embodiment of the presentinvention.

FIGS. 15A to 15C each illustrate an electronic appliance including adisplay device of one embodiment of the present invention.

FIGS. 16A to 16C illustrate an electronic appliance including a displaydevice of one embodiment of the present invention.

FIGS. 17A to 17C show the relation between current and voltage of atransistor included in a display device and the relation between voltageand capacitance of a capacitor in the display device.

FIG. 18 shows a method for operating a capacitor included in a displaydevice.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways. Therefore, the present inventionis not construed as being limited to description of the embodimentsbelow.

In structures of the present invention described below, the sameportions or portions having similar functions are denoted by the samereference numerals in different drawings, and description thereof is notrepeated. Further, the same hatching pattern is applied to portionshaving similar functions, and the portions are not especially denoted byreference numerals in some cases.

In each drawing described in this specification, the size, the filmthickness, or the region of each component is exaggerated for clarity insome cases. Therefore, embodiments of the present invention are notlimited to such scales.

The ordinal numbers such as “first” and “second” in this specificationand the like are used for convenience and do not indicate the order ofsteps or the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

Functions of a “source” and a “drain” in the present invention aresometimes replaced with each other when the direction of a currentflowing is changed in circuit operation, for example. Therefore, theterms “source” and “drain” can be used to denote the drain and thesource, respectively, in this specification.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential (e.g., a ground potential or a sourcepotential) is merely called a potential or voltage, and “potential” and“voltage” are used as synonymous words. Thus, in this specification, apotential may be rephrased as a voltage and a voltage may be rephrasedas a potential unless otherwise specified.

In this specification, in the case where etching treatment is performedafter a photolithography process, a mask formed by the photolithographyprocess is removed.

Embodiment 1

In this embodiment, a display device which is one embodiment of thepresent invention is described with reference to drawings. Note that inthis embodiment, a display device including a liquid crystal element isdescribed as an example.

<Structure of Display Device>

FIG. 1A illustrates an example of a display device. The display deviceillustrated in FIG. 1A includes a pixel portion 100; a scan line drivercircuit 104; a signal line driver circuit 106; m scan lines 107 whichare arranged in parallel or almost in parallel to each other and whosepotentials are controlled by the scan line driver circuit 104; and nsignal lines 109 which are arranged in parallel or almost in parallel toeach other and whose potentials are controlled by the signal line drivercircuit 106. The pixel portion 100 includes a plurality of pixels 101arranged in matrix. Capacitor lines 115 which are arranged in parallelor almost in parallel to the scan lines 107 are also provided. Thecapacitor lines 115 may be arranged in parallel or almost in parallel tothe signal lines 109.

Each scan line 107 is electrically connected to the n pixels 101 in thecorresponding row among the pixels 101 arranged in m rows and n columnsin the pixel portion 100. Each signal line 109 is electrically connectedto the m pixels 101 in the corresponding column among the pixels 101arranged in m rows and n columns. Note that m and n are each an integerof 1 or more. Each capacitor line 115 is electrically connected to the npixels 101 in the corresponding row among the pixels 101 arranged in mrows and n columns. Note that in the case where the capacitor lines 115are arranged in parallel or almost in parallel to the signal lines 109,each capacitor line 115 is electrically connected to the m pixels 101 inthe corresponding column among the pixels 101 arranged in m rows and ncolumns.

FIG. 1B is an example of a circuit diagram of the pixel 101 included inthe display device illustrated in FIG. 1A. The pixel 101 illustrated inFIG. 1B includes a transistor 103, a capacitor 105, and a liquid crystalelement 108. The transistor 103 is electrically connected to the scanline 107 and the signal line 109. One electrode of the capacitor 105 iselectrically connected to a drain electrode of the transistor 103 and apixel electrode 121, and the other electrode of the capacitor 105 iselectrically connected to the capacitor line 115 which supplies aconstant potential. The liquid crystal element 108, in which the pixelelectrode 121 is electrically connected to the drain electrode of thetransistor 103 and the one electrode of the capacitor 105, iselectrically connected to a wiring 155 which supplies a potential to anelectrode (counter electrode) facing the pixel electrode 121.

In the capacitor 105, part of the pixel electrode 121 can function asone electrode and the electrode connected to the capacitor line 115 canfunction as the other electrode. At this time, in the case where theconductivity of the other electrode is high, the capacitor 105 can beillustrated as in the circuit diagram of FIG. 1B. On the other hand, inthe case where the conductivity of the other electrode is low, thecapacitor 105 can be illustrated as in the circuit diagram of FIG. 1C.

The pixel 101 illustrated in FIG. 1C includes a transistor 103, acapacitor 105, and a liquid crystal element 108. The transistor 103 iselectrically connected to the scan line 107 and the signal line 109. Oneelectrode of the capacitor 105 is electrically connected to a drainelectrode of the transistor 103 and a pixel electrode 121, and the otherelectrode of the capacitor 105 is electrically connected to thecapacitor line 115 which supplies a constant potential. The liquidcrystal element 108, in which the pixel electrode 121 is electricallyconnected to the drain electrode of the transistor 103 and the oneelectrode of the capacitor 105, is electrically connected to the wiring155 which supplies a potential to an electrode (counter electrode)facing the pixel electrode 121.

In the capacitor 105 illustrated in FIG. 1C, a multilayer film 119 hasthe same layer structure as a multilayer film 111 used for thetransistor 103. The multilayer film 119 functions as an electrode whenthe multilayer film 119 is brought into a conductive state bycontrolling a potential to be supplied thereto. The multilayer film 119functions as the other electrode of the capacitor 105. Thus, thecapacitor 105 can be said to have a metal oxide semiconductor (MOS)capacitor structure.

The multilayer film 119 included in the capacitor 105 is formed byutilizing the formation process of the multilayer film 111 included inthe transistor 103 which is an enhancement-mode transistor. Thus, thecapacitor 105 starts to be charged when a difference between a potentialVP of the pixel electrode 121 and a potential VC of the capacitor line115 (VP−VC) is higher than or equal to 0 V (see FIGS. 17B and 17C).Thus, the threshold voltage (Vth) of the capacitor 105 is higher than orequal to 0 V. FIG. 17A shows an I-V curve of the enhancement-modetransistor 103 and threshold voltage thereof. Note that the verticalaxis of FIG. 17A is a logarithmic axis.

FIG. 17B shows a C-V (capacitance-voltage) curve of the capacitor 105.In FIG. 17B, the horizontal axis represents a potential difference(VP−VC) between the pixel electrode 121 of the capacitor 105 and thecapacitor line 115, and the vertical axis represents capacitance C ofthe capacitor 105. In the case where the frequency of voltage at thetime of the C-V measurement is lower than the frame frequency of thedisplay device, a C-V curve as shown in FIG. 17B is obtained.

The multilayer film 119 of the capacitor 105 can be formed by utilizingthe formation process of the multilayer film 111 of the transistor 103;thus the multilayer film 119 is an oxide semiconductor film which is notsubjected to treatment for intentionally increasing carrier density,treatment for adding an impurity for increasing conductivity, or thelike. Thus, the carrier density of the multilayer film 119 is equivalentto that of the multilayer film 111.

The liquid crystal element 108 is an element which controls transmissionof light by an optical modulation action of liquid crystal which issandwiched between a substrate provided with the transistor 103 and thepixel electrode 121 and a substrate provided with the counter electrode.The optical modulation action of a liquid crystal is controlled by anelectric field applied to the liquid crystal (including a horizontalelectric field, a vertical electric field, and an oblique electricfield).

Next, a specific example of the pixel 101 of the liquid crystal displaydevice is described. FIG. 2 is a top view of the pixel 101. Note that inFIG. 2, the counter electrode and the liquid crystal element areomitted.

In FIG. 2, the scan line 107 extends in a direction substantiallyperpendicular to the signal line 109 (in the horizontal direction in thedrawing). The signal line 109 extends in a direction substantiallyperpendicular to the scan line 107 (in the vertical direction in thedrawing). The capacitor line 115 extends in a direction parallel to thescan line 107. Note that the scan line 107 and the capacitor line 115are electrically connected to the scan line driver circuit 104 (see FIG.1A), and the signal line 109 is electrically connected to the signalline driver circuit 106 (see FIG. 1A).

The transistor 103 is provided in a region where the scan line 107 andthe signal line 109 cross each other. The transistor 103 includes a gateelectrode, a gate insulating film (not illustrated in FIG. 2) over thegate electrode, and the multilayer film 111 including an oxidesemiconductor layer over the gate insulating film. A region of the scanline 107 which overlaps with the multilayer film 111 functions as thegate electrode of the transistor 103. A region of the scan line 109which overlaps with the multilayer film 111 functions as a sourceelectrode of the transistor 103. A region of a conductive film 113 whichoverlaps with the multilayer film 111 functions as the drain electrodeof the transistor 103. Thus, the gate electrode, the source electrode,and the drain electrode may be referred to as the scan line 107, thesignal line 109, and the conductive film 113, respectively. Further, inFIG. 2, an edge of the scan line 107 is on the outer side of an edge ofthe multilayer film 111 when seen from above. Thus, the scan line 107functions as a light-blocking film for blocking light from a backlight.For this reason, the multilayer film 111 including an oxidesemiconductor layer which is included in the transistor 103 is notirradiated with light, so that variation in the electricalcharacteristics of the transistor 103 can be reduced.

Further, when the multilayer film 111 including the oxide semiconductorlayer is processed under appropriate conditions, the off-state currentof the transistor 103 can be extremely low. This enables the powerconsumption of the display device to be reduced.

In this embodiment, a structure of the multilayer film 111 including theoxide semiconductor layer which includes a first oxide layer, a secondoxide layer which is an oxide semiconductor over and in contact with thefirst oxide layer, and a third oxide layer over and in contact with thesecond oxide layer is described.

The conductive film 113 is electrically connected to thelight-transmitting pixel electrode 121 through an opening 117. Thus, thelight-transmitting pixel electrode 121 is electrically connected to thetransistor 103.

The capacitor 105 is provided in a region which is in the pixel 101 andis surrounded by the capacitor lines 115 and the signal lines 109. Notethat the capacitor 105 has a light-transmitting property. Thus, thecapacitor 105 can be formed large (in a large area) in the pixel 101.This enables the display device to have a higher aperture ratio andhigher charge capacity.

For example, in a high-definition display device such as a liquidcrystal display device, the area of a pixel is small and accordingly thearea of a capacitor is also small. For this reason, the capacity ofcharge stored in the capacitor 105 is small in the high-definitiondisplay device. However, since the capacitor 105 described in thisembodiment has a light-transmitting property, when the capacitor 105 isprovided in each pixel, enough charge capacity can be obtained in eachpixel and the aperture ratio can be improved. Typically, the capacitor105 can be favorably used for a high-definition display device with apixel density of 200 pixels per inch (ppi) or more, or furthermore, 300ppi or more. Further, according to one embodiment of the presentinvention, the aperture ratio can be improved, which makes it possibleto use efficiently light from a light source device such as a backlight,so that the power consumption of the display device can be reduced.

Now, a method for driving the display device of one embodiment of thepresent invention is described. Since the display device of oneembodiment of the present invention includes the capacitor 105 having aMOS capacitor structure, the potential VC is supplied to the multilayerfilm 119 functioning as the other electrode of the capacitor 105 (i.e.,the capacitor line 115) as described below so that the capacitor 105 isstably operated.

As described above, the C-V curve of the capacitor 105 is a C-V curveshowing a threshold voltage higher than or equal to 0 V as shown in FIG.17B. The capacitor 105 is stably operated in an operation period of thecapacitor 105 means that the capacitor 105 is sufficiently charged. Itmeans that, for example, the potential VC is supplied so that thedifference between the potential VP of the pixel electrode 121 of thecapacitor 105 and the potential VC of the multilayer film 119 (VP−VC) inthe operation period of the capacitor 105 is higher than or equal to V1and lower than or equal to V2 in FIG. 17B.

In the operation period of the capacitor 105, the potential VP of thepixel electrode 121 shifts in the positive direction and the negativedirection depending on a signal input to the signal line 109.Specifically, the potential VP shifts in the positive direction and thenegative direction relative to the central potential of a video signal.Thus, in order that the potential difference between the pixel electrode121 and the multilayer film 119 is set to V1 and V2 in the operationperiod of the capacitor 105, the potential of the multilayer film 119may be a potential lower than each of V1 and V2 by the threshold voltageof the capacitor 105 or more (see FIG. 18). Note that in FIG. 18, thelowest potential of the potentials supplied to the scan line 107 is GVssand the highest potential thereof is GVdd.

In other words, in order to operate the capacitor 105, the potentialdifference between the pixel electrode 121 and the multilayer film 119is preferably higher than the threshold voltage of the capacitor 105 inthe operation period of the capacitor 105.

Further, the multilayer film 119 of the capacitor 105 has the samestructure as the multilayer film 111 of the transistor 103; thus, thethreshold voltage of the capacitor 105 is equivalent to that of thetransistor 103. Accordingly, the potential VC of the multilayer film 119is preferably set to a potential which is lower than or equal to apotential obtained by subtracting the threshold voltage of thetransistor 103 from the potential VP of the pixel electrode 121. In thismanner, in an operation period of the capacitor 105, the multilayer film119 can be constantly in a conductive state, so that the capacitor 105can be operated stably.

As described above, by using a driving method which is one embodiment ofthe present invention, a display device provided with a capacitorincluding a light-transmitting semiconductor film, a light-transmittingconductive film, and a light-transmitting insulating film can beoperated while the capacitor is stable with time.

Further, the transistor 103 is an enhancement-mode transistor and thecapacitor 105 is formed by utilizing a formation process of thetransistor 103 which is an enhancement-mode transistor; thus, thevoltage range needed for driving the display device which is oneembodiment of the present invention is narrower than that needed fordriving a display device in which a depletion-mode transistor is used asthe transistor 103 and the capacitor 105 is formed using an oxidesemiconductor film in which carrier density is increased and which isformed by utilizing a formation process of the depletion-modetransistor. Thus, according to one embodiment of the present invention,power consumption of the display device can be reduced.

FIG. 3A is a cross-sectional view taken along the dashed-dotted linesA1-A2 and B1-B2 in FIG. 2.

A cross-sectional structure of the pixel 101 of the liquid crystaldisplay device in FIG. 3A is as described below. The liquid crystaldisplay device includes an element portion over a first substrate 102,an element portion on a second substrate 150, and a liquid crystal layer160 interposed between the two element portions.

First, a structure of the element portion over the first substrate 102is described. The scan line 107 part of which functions as the gateelectrode of the transistor 103 and the capacitor line 115 over the samesurface as the scan line 107 are provided over the first substrate 102.A gate insulating film 127 is provided over the scan line 107 and thecapacitor line 115. The multilayer film 111 is provided in a region overthe gate insulating film 127 which overlaps with the scan line 107, andthe multilayer film 119 is provided over the gate insulating film 127 ina region where the capacitor 105 is formed. The signal line 109 part ofwhich functions as the source electrode of the transistor 103 and theconductive film 113 part of which functions as the drain electrode ofthe transistor 103 are provided over the gate insulating film 127.

An opening 123 reaching the capacitor line 115 is formed in the gateinsulating film 127 on which the capacitor 105 is formed, and theconductive film 125 is provided in the opening 123 and over the gateinsulating film 127 and the multilayer film 119.

Further, an insulating film 129, an insulating film 131, and aninsulating film 132 which function as protective insulating films of thetransistor 103 and dielectrics of the capacitor 105 are provided overthe gate insulating film 127, the signal line 109, the multilayer film111, the conductive film 113, the conductive film 125, and themultilayer film 119. Note that the opening 117 reaching the conductivefilm 113 is formed in the insulating film 129, the insulating film 131,and the insulating film 132, and the pixel electrode 121 is provided inthe opening 117 and over the insulating film 132.

The capacitor 105 has a light-transmitting property and includes thepixel electrode 121, the insulating film 129, the insulating film 131,the insulating film 132, and the multilayer film 119.

An insulating film 158 functioning as an alignment film is provided overthe pixel electrode 121 and the insulating film 132. Note that a baseinsulating film may be provided between the first substrate 102 and thescan line 107, the capacitor line 115, and the gate insulating film 127.

FIG. 3B is an enlarged view of a region α (the gate insulating film 127,the multilayer film 111, the signal line 109, and the insulating film129) of the display device illustrated in FIG. 3A. FIG. 3C is anenlarged view of a region β (the gate insulating film 127, themultilayer film 119, the conductive film 125, and the insulating film129) of the display device illustrated in FIG. 3A.

In FIG. 3B, the multilayer film 111 includes a first oxide layer 111_1in contact with the gate insulating film 127, a second oxide layer 111_2which is an oxide semiconductor over and in contact with the first oxidelayer 111_1, and a third oxide layer 111_3 over and in contact with thesecond oxide layer 111_2. Note that the signal line 109 and theinsulating film 129 are formed over the third oxide layer 111_3. Thethickness of the first oxide layer 111_1 is greater than or equal to 1nm and less than or equal to 50 nm, preferably greater than or equal to5 nm and less than or equal to 50 nm, more preferably greater than orequal to 10 nm and less than or equal to 40 nm. The thickness of thesecond oxide layer 111_2 is greater than or equal to 1 nm and less thanor equal to 50 nm, preferably greater than or equal to 3 nm and lessthan or equal to 40 nm, more preferably greater than or equal to 5 nmand less than or equal to 30 nm. The thickness of the third oxide layer111_3 is greater than or equal to 1 nm and less than or equal to 50 nm,preferably greater than or equal to 3 nm and less than or equal to 40nm, more preferably greater than or equal to 5 nm and less than or equalto 30 nm.

The first oxide layer 111_1 is an oxide film which is formed of one ormore kinds of elements other than oxygen, which form the second oxidelayer 111_2 and has a lower electron affinity than the second oxidelayer 111_2 by 0.2 eV or more. At this time, when an electric field isapplied to the gate electrode, a channel is formed in the second oxidelayer 111_2 having a higher electron affinity than the other layers inthe multilayer film 111 including the oxide semiconductor layer. Inother words, the first oxide layer 111_1 is provided between the secondoxide layer 111_2 and the gate insulating film 127, so that the channelof the transistor 103 can be formed in a layer (here, the second oxidelayer 111_2) which is not in contact with the gate insulating film 127.

The first oxide layer 111_1 may be, for example, an oxide filmcontaining aluminum, silicon, titanium, gallium, germanium, yttrium,zirconium, tin, lanthanum, cerium, or hafnium at a concentration higherthan that in the second oxide layer 111_2. Specifically, as the firstoxide layer 111_1, an oxide film containing the above element at aconcentration 1.5 times or more, preferably twice or more, morepreferably 3 times or more that in the second oxide layer 111_2 is used.The above element is strongly bonded to oxygen, and thus has a functionof suppressing generation of oxygen vacancies in the oxide film. Inother words, the first oxide layer 111_1 is an oxide film in whichoxygen vacancies are less likely to be generated than in the secondoxide layer 111_2.

Alternatively, in the case where the second oxide layer 111_2 is anIn—Ga—Zn-based oxide film having an atomic ratio of In:Ga:Zn=x₂:y₂:z₂and the first oxide layer 111_1 is also an In—Ga—Zn-based oxide filmhaving an atomic ratio of In:Ga:Zn=x₁:y₁:z₁, the first oxide layer 111_1and the second oxide layer 111_2 which satisfy the following conditionsare selected: y₁/x₁ is larger than y₂/x₂, preferably y₁/x₁ is 1.5 timesor more as large as y₂/x₂, more preferably y₁/x₁ is twice or more aslarge as y₂/x₂, further preferably y₁/x₁ is three times or more as largeas y₂/x₂.

The third oxide layer 111_3 is an oxide film which is formed of one ormore kinds of elements other than oxygen, which form the second oxidelayer 111_2 and has a lower electron affinity than the second oxidelayer 111_2 by 0.2 eV or more. At this time, a channel is not formed inthe third oxide layer 111_3 even when an electric field is applied tothe gate electrode. Further, since the third oxide layer 111_3 is formedof one or more kinds of elements other than oxygen, which form thesecond oxide layer 111_2, an interface state is less likely to be formedat an interface between the second oxide layer 111_2 and the third oxidelayer 111_3. When the interface has an interface state, a secondtransistor in which the interface serves as a channel formation regionis formed and the apparent threshold voltage of the transistor varies insome cases. Thus, providing the third oxide layer 111_3 makes itpossible to reduce variation in the electrical characteristics of thetransistor, such as threshold voltage.

The third oxide layer 111_3 may be, for example, an oxide filmcontaining aluminum, silicon, titanium, gallium, germanium, yttrium,zirconium, tin, lanthanum, cerium, or hafnium at a concentration higherthan that in the second oxide layer 111_2. Specifically, as the thirdoxide layer 111_3, an oxide film containing the above element at aconcentration 1.5 times or more, preferably twice or more, morepreferably 3 times or more that in the second oxide layer 111_2 is used.The above element is strongly bonded to oxygen, and thus has a functionof suppressing generation of oxygen vacancies in the oxide film. Inother words, the third oxide layer 111_3 is an oxide film in whichoxygen vacancies are less likely to be generated than in the secondoxide layer 111_2.

Alternatively, in the case where the second oxide layer 111_2 is anIn—Ga—Zn-based oxide film having an atomic ratio of In:Ga:Zn=x₂:y₂:z₂and the third oxide layer 111_3 is also an In—Ga—Zn-based oxide filmhaving an atomic ratio of In:Ga:Zn=x₃:y₃:z₃, the second oxide layer111_2 and the third oxide layer 111_3 which satisfy the followingconditions are selected: y₃/x₃ is larger than y₂/x₂, preferably, y₃/x₃is 1.5 times or more as large as y₂/x₂, more preferably y₃/x₃ is twiceor more as large as y₂/x₂, further preferably y₃/x₃ is 3 times or moreas large as y₂/x₂.

Oxide semiconductors with different crystallinity may be applied to thefirst oxide layer 111_1 to the third oxide layer 111_3. That is, thefirst oxide layer 111_1 to the third oxide layer 111_3 may each have astructure in which an oxide semiconductor in which a crystal part is notclearly observed, such as an amorphous oxide semiconductor, and an oxidesemiconductor having a crystalline structure (a crystalline oxidesemiconductor) such as a single-crystal oxide semiconductor, apolycrystalline oxide semiconductor, and a CAAC-OS (Embodiment 4 isreferred to for the details) are combined as appropriate. When an oxidesemiconductor in which a crystal part is not clearly observed, such asan amorphous oxide semiconductor, is used as any one of the first tothird oxide semiconductor layers 111_1 to 111_3, internal stress orexternal stress of the oxide semiconductor film is relieved, variationsin characteristics of a transistor is reduced and a variation in thethreshold voltage of the transistor due to a change over time or areliability test can be reduced.

For example, the first oxide layer 111_1 is preferably an oxidesemiconductor in which a crystal part is not clearly observed, such asan amorphous oxide semiconductor. Further, the second oxide layer 111_2which can be a channel formation region is preferably a crystallineoxide semiconductor. Further, the third oxide layer 111_3 is preferablyan oxide semiconductor in which a crystal part is not clearly observed,such as an amorphous oxide semiconductor or a crystalline oxidesemiconductor. Such a structure enables the amount of change in thethreshold voltage of the transistor due to change over time or areliability test to be reduced.

In FIG. 3C, the multilayer film 119 includes a first oxide layer 119_1which is in contact with the gate insulating film 127, a second oxidelayer 119_2 which is an oxide semiconductor film over and in contactwith the first oxide layer 119_1, and a third oxide layer 119_3 which isover and in contact with the second oxide layer 119_2. Note that theconductive film 125 and the insulating film 129 are formed over thethird oxide layer 119_3.

The multilayer film 119 functioning as the other electrode of thecapacitor 105 has the same layer structure as the multilayer film 111.In other words, the multilayer film 119 can be formed using an oxidefilm which can be applied to the multilayer film 111. Further, since themultilayer film 119 can be formed together with the multilayer film 111,the multilayer film 119 contains a metal element of an oxidesemiconductor forming the multilayer film 111.

The details of the components of the above-described structure aredescribed below.

Although there is no particular limitation on the material and the likeof the first substrate 102, it is necessary that the first substrate 102have heat resistance high enough to withstand at least heat treatmentperformed in a manufacturing process of a display device. Examples ofthe substrate are a glass substrate, a ceramic substrate, and a plasticsubstrate, and as the glass substrate, an alkali-free glass substratesuch as a barium borosilicate glass substrate, an aluminoborosilicateglass substrate, or an aluminosilicate glass substrate is preferablyused. Alternatively, a non-light-transmitting substrate such as astainless alloy substrate may be used. In this case, a surface of thesubstrate is preferably provided with an insulating film. As the firstsubstrate 102, any of the following substrates may alternatively beused: a quartz substrate, a sapphire substrate, a single crystalsemiconductor substrate, a polycrystalline semiconductor substrate, acompound semiconductor substrate, and a silicon on insulator (SOI)substrate.

The scan line 107 and the capacitor line 115, through which a largeamount of current flows, are preferably formed using a metal film;typically, they are formed to have a single-layer structure or astacked-layer structure using any of metal materials such as molybdenum(Mo), titanium (Ti), tungsten (W), tantalum (Ta), aluminum (Al), copper(Cu), chromium (Cr), neodymium (Nd), or scandium (Sc), or an alloymaterial which contains any of these materials as its main component.

Examples of the scan line 107 and the capacitor line 115 are asingle-layer structure using aluminum containing silicon, a two-layerstructure in which titanium is stacked over aluminum, a two-layerstructure in which titanium is stacked over titanium nitride, atwo-layer structure in which tungsten is stacked over titanium nitride,a two-layer structure in which tungsten is stacked over tantalumnitride, a two-layer structure in which copper is stacked over Cu—Mg—Alalloy, and a three-layer structure in which titanium nitride, copper,and tungsten are stacked in this order.

As a material of the scan line 107 and the capacitor line 115, any ofthe light-transmitting conductive materials described later, which canbe used for the pixel electrode 121, can be used.

Alternatively, as a material of the scan line 107 and the capacitor line115, a metal oxide containing nitrogen, specifically, an In—Ga—Zn-basedoxide containing nitrogen, an In—Sn-based oxide containing nitrogen, anIn—Ga-based oxide containing nitrogen, an In—Zn-based oxide containingnitrogen, a Sn-based oxide containing nitrogen, an In-based oxidecontaining nitrogen, or a metal nitride (e.g., InN or SnN) can be used.These materials each have a work function of 5 eV or higher. The use ofthe metal oxide containing nitrogen for the scan line 107 (the gateelectrode of the transistor 103) allows the threshold voltage of thetransistor 103 to shift in the positive direction, that is, anenhancement-mode transistor is easily achieved. For example, in the casewhere an In—Ga—Zn-based oxide containing nitrogen is used, anIn—Ga—Zn-based oxide having a higher nitrogen concentration than atleast the multilayer film 111 including the oxide semiconductor layer;specifically an In—Ga—Zn-based oxide having a nitrogen concentration of7 at. % or higher can be used.

The scan line 107 and the capacitor line 115 are preferably formed usingaluminum or copper, which are low resistance materials. With the use ofaluminum or copper, signal delay can be reduced, so that the imagequality of the display device can be improved. Note that aluminum haslow heat resistance, and thus a defect due to hillocks, whiskers, ormigration is easily generated. In order to prevent a defect due tomigration of aluminum, a layer of a metal material having a highermelting point than aluminum, such as molybdenum, titanium, or tungsten,is preferably stacked over an aluminum layer. Also in the case wherecopper is used, in order to prevent a defect due to migration and thelike and diffusion of copper elements, a layer of a metal materialhaving a higher melting point than copper, such as molybdenum, titanium,or tungsten, is preferably stacked over a copper layer.

The gate insulating film 127 is formed to have a single-layer structureor a stacked-layer structure using, for example, one or more ofinsulating materials such as silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, aluminum oxide, gallium oxide, and aGa—Zn-based oxide.

Further, it is possible to prevent outward diffusion of oxygen from themultilayer film 111 including the oxide semiconductor layer and entry ofhydrogen, water, or the like into the multilayer film 111 including theoxide semiconductor layer from the outside by providing an insulatingfilm having a bather property against oxygen, hydrogen, water, and thelike for the gate insulating film 127. Examples of the insulating filmhaving a barrier property against oxygen, hydrogen, water, and the likeare an aluminum oxide film, an aluminum oxynitride film, a gallium oxidefilm, a gallium oxynitride film, an yttrium oxide film, an yttriumoxynitride film, and a silicon nitride film.

The gate insulating film 127 preferably has the stacked-layer structuredescribed below. It is preferable that a silicon nitride film havingfewer defects be provided as a first silicon nitride film, a siliconnitride film from which less hydrogen and ammonia are released beprovided as a second silicon nitride film over the first silicon nitridefilm, and any of the oxide insulating films listed as those used for thegate insulating film 127 be provided over the second silicon nitridefilm.

As the second silicon nitride film, a nitride insulating film in whichthe number of released hydrogen molecules is preferably less than 5×10²¹molecules/cm³, more preferably less than or equal to 3×10²¹molecules/cm³, further preferably less than or equal to 1×10²¹molecules/cm³, and the number of released ammonia molecules ispreferably less than 1×10²² molecules/cm³, more preferably less than orequal to 5×10²¹ molecules/cm³, further preferably less than or equal to1×10²¹ molecules/cm³ when measured by thermal desorption spectroscopy ispreferably used. The first silicon nitride film and the second siliconnitride film are used as part of the gate insulating film 127, so that agate insulating film with a small number of defects and small amounts ofreleased hydrogen and released ammonia can be formed as the gateinsulating film 127. Thus, the amount of hydrogen and nitrogen containedin the gate insulating film 127 which enter the multilayer film 111including the oxide semiconductor layer can be reduced.

The thickness of the gate insulating film 127 is greater than or equalto 5 nm and less than or equal to 400 nm, preferably greater than orequal to 10 nm and less than or equal to 300 nm, more preferably greaterthan or equal to 50 nm and less than or equal to 250 nm.

The multilayer film 111 and the multilayer film 119 each including theoxide semiconductor layer can have an amorphous structure, asingle-crystal structure, or a polycrystalline structure. Thethicknesses of the multilayer film 111 and the multilayer film 119 eachincluding the oxide semiconductor layer are greater than or equal to 1nm and less than or equal to 100 nm, preferably greater than or equal to1 nm and less than or equal to 50 nm, more preferably greater than orequal to 3 nm and less than or equal to 40 nm, further preferablygreater than or equal to 5 nm and less than or equal to 30 nm.

An oxide semiconductor which can be used for the multilayer film 111including the oxide semiconductor layer and the multilayer film 119including the oxide semiconductor layer has an energy gap greater thanor equal to 2.5 eV, preferably greater than or equal to 2.7 eV, morepreferably greater than or equal to 3 eV. The use of such an oxidesemiconductor having a wide energy gap can reduce the off-state currentof the transistor 103.

For the multilayer film 111 including the oxide semiconductor layer andthe multilayer film 119 including the oxide semiconductor layer, forexample, any of the following can be used: an indium oxide; a tin oxide;a zinc oxide; an oxide containing two kinds of metals, such as anIn—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, aZn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or anIn—Ga-based oxide; an oxide containing three kinds of metals, such as anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, anAl—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Zr—Zn-based oxide,an In—Ti—Zn-based oxide, an In—Sc—Zn-based oxide, an In—Y—Zn-basedoxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, anIn—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide,an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-basedoxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, anIn—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide,an In—Lu—Zn-based oxide, or an In—Hf—Zn-based oxide; and an oxidecontaining four kinds of oxides, such as an In—Sn—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, or an In—Sn—Al—Zn-based oxide.

Here, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Znas its main components and there is no particular limitation on theatomic ratio of In:Ga:Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0) may beused as an oxide semiconductor. Note that M represents one or more metalelements selected from Ga, Fe, Mn, and Co.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1, 2:2:1, or 3:1:2 can be used. Alternatively, anIn—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1, 2:1:3, or2:1:5 can be used. Note that the proportion of each atom in the atomicratio of the metal oxide varies within a range of ±20% as an error.

Here, the characteristics of a transistor including an oxidesemiconductor are described. Note that the transistor including an oxidesemiconductor used in one embodiment of the present invention is ann-channel transistor. Oxygen vacancies in an oxide semiconductor mightgenerate carriers, which might lower the electrical characteristics andreliability of the transistor. For example, in some cases, the thresholdvoltage of the transistor shifts in the negative direction, and draincurrent flows when the gate voltage is 0 V.

In view of the above, it is preferable that defects, typically oxygenvacancies, in the multilayer film 111 including the oxide semiconductorlayer be reduced as much as possible. For example, it is preferable thatthe spin density of the oxide semiconductor film (the density of defectsin the oxide semiconductor film) at a g-value higher than or equal to1.89 and lower than or equal to 1.96 (typically, 1.93) in electron spinresonance spectroscopy in which a magnetic field is applied in parallelto the film surface be reduced to lower than or equal to the lowerdetection limit of measurement equipment. When the defects, typicallyoxygen vacancies, in the oxide semiconductor film are reduced as much aspossible, the transistor 103 can be prevented from being adepletion-mode transistor, leading to improvements in the electricalcharacteristics and reliability of the display device.

The shift of the threshold voltage of a transistor in the negativedirection is caused in some cases by hydrogen (including a hydrogencompound such as water) contained in an oxide semiconductor as well asby oxygen vacancies. Hydrogen contained in the oxide semiconductor isreacted with oxygen bonded to a metal atom to be water, and in addition,vacancies (also referred to as oxygen vacancies) are formed in a latticefrom which oxygen is released (or a portion from which oxygen isremoved). In addition, when part of hydrogen reacts with oxygen,electrons serving as carriers are generated. Thus, a transistorincluding an oxide semiconductor which contains hydrogen is likely to bea depletion-mode transistor.

In view of the above, it is preferable that hydrogen in the multilayerfilm 111 including the oxide semiconductor layer be reduced as much aspossible. Specifically, in the multilayer film 111 including the oxidesemiconductor layer, the concentration of hydrogen which is measured bysecondary ion mass spectrometry (SIMS) is set to lower than 5×10¹⁸atoms/cm³, preferably lower than or equal to 1×10¹⁸ atoms/cm³, morepreferably lower than or equal to 5×10¹⁷ atoms/cm³, still morepreferably lower than or equal to 1×10¹⁶ atoms/cm³.

Further, in the multilayer film 111 including the oxide semiconductorlayer, the concentration of alkali metals or alkaline earth metals whichis measured by SIMS is set to lower than or equal to 1×10¹⁸ atoms/cm³,preferably lower than or equal to 2×10¹⁶ atoms/cm³. This is because analkali metal and an alkaline earth metal might generate carriers whenbonded to an oxide semiconductor, in which case the off-state current ofthe transistor 103 might be increased.

Further, when nitrogen is contained in the multilayer film 111 includingthe oxide semiconductor layer, electrons serving as carriers aregenerated to increase the carrier density, so that the multilayer film111 easily becomes n-type. Thus, a transistor including an oxidesemiconductor which contains nitrogen is likely to be a depletion-modetransistor. For this reason, nitrogen in the multilayer film 111including the oxide semiconductor layer is preferably reduced as much aspossible; the concentration of nitrogen is preferably set to, forexample, lower than or equal to 5×10¹⁸ atoms/cm³.

In this manner, when the multilayer film 111 including the oxidesemiconductor layer which is highly purified by reducing impurities(e.g., hydrogen, nitrogen, an alkali metal, and an alkaline earth metal)as much as possible is used, the transistor 103 can be prevented frombeing a depletion-mode transistor, so that the off-state current of thetransistor 103 can be significantly reduced. Thus, a display devicehaving favorable electrical characteristics can be manufactured.Further, a display device with improved reliability can be manufactured.

Various experiments can prove low off-state current of a transistorincluding a highly-purified oxide semiconductor film. For example, evenwhen an element has a channel width of 1×10⁶ μm and a channel length (L)of 10 μm, the off-state current can be lower than or equal to themeasurement limit of a semiconductor parameter analyzer, i.e., less thanor equal to 1×10⁻¹³ A, at a voltage (drain voltage) between a sourceelectrode and a drain electrode of from 1 V to 10 V. In this case, itcan be seen that the off-state current corresponding to a value obtainedby dividing the off-state current by the channel width of the transistoris 100 zA/μm or lower. In addition, a capacitor and a transistor areconnected to each other and the off-state current is measured with acircuit in which charge flowing into or from the capacitor is controlledby the transistor. In the measurement, a highly purified oxidesemiconductor film is used for a channel formation region of thetransistor, and the off-state current of the transistor is measured froma change in the amount of charge of the capacitor per unit time. As aresult, it is found that in the case where the voltage between thesource electrode and the drain electrode of the transistor is 3 V, loweroff-state current of several tens of yoctoamperes per micrometer (yA/μm)can be obtained. Thus, the transistor including the highly purifiedoxide semiconductor film has a significantly low off-state current.

The signal line 109 part of which functions as the source electrode ofthe transistor 103, the conductive film 113 part of which functions asthe drain electrode of the transistor 103, and the conductive film 125electrically connecting the multilayer film 119 of the capacitor 105 tothe capacitor line 115 can be formed to have a single-layer structure ora stacked-layer structure using a material which can be used for thescan line 107 and the capacitor line 115.

The insulating films 129, 131, and 132 functioning as the protectiveinsulating films of the transistor 103 and the dielectric films in thecapacitor 105 are insulating films each of which is formed using amaterial which can be used for the gate insulating film 127. It isparticularly preferable that the insulating films 129 and 131 be oxideinsulating films and the insulating film 132 be a nitride insulatingfilm. Further, the use of a nitride insulating film as the insulatingfilm 132 can suppress entry of impurities such as hydrogen and waterinto the transistor 103 (particularly the multilayer film 111 includingthe oxide semiconductor layer) from the outside. Note that theinsulating film 129 is not necessarily provided.

Further, an oxide insulating film in which the oxygen content is higherthan that in the stoichiometric composition is preferably used as one orboth of the insulating film 129 and the insulating film 131. In thatcase, oxygen can be prevented from being released from the oxidesemiconductor layer, and the oxygen contained in an oxygen excess regioncan be transferred to the oxide semiconductor layer to fill oxygenvacancies. For example, when an oxide insulating film having the featuredescribed below is used, the oxygen vacancies in the oxide semiconductorlayer can be filled. The feature of the oxide insulating film is thatthe number of oxygen molecules released from the oxide insulating filmis greater than or equal to 1.0×10¹⁸ molecules/cm³ when measured bythermal desorption spectroscopy (hereinafter referred to as TDS). Notethat an oxide insulating film partly including a region in which theoxygen content is higher than that in the stoichiometric composition(oxygen excess region) may be used as one or both of the insulating film129 and the insulating film 131. When such an oxygen excess region ispresent in a region overlapping with at least the multilayer film 111including the oxide semiconductor layer, oxygen is prevented from beingreleased from the oxide semiconductor layer and the oxygen contained inthe oxygen excess region can be transferred to the oxide semiconductorlayer to fill oxygen vacancies.

In the case where the insulating film 131 is an oxide insulating film inwhich the oxygen content is higher than that in the stoichiometriccomposition, the insulating film 129 is preferably an oxide insulatingfilm through which oxygen penetrates. Oxygen which enters the insulatingfilm 129 from the outside does not completely penetrate through theinsulating film 129 to be released and part thereof remains in theinsulating film 129. Further, there is oxygen which is contained in theinsulating film 129 from the first and is released from the insulatingfilm 129 to the outside. Thus, the insulating film 129 preferably has ahigh coefficient of diffusion of oxygen.

The thickness of the insulating film 129 can be greater than or equal to5 nm and less than or equal to 150 nm, preferably greater than or equalto 5 nm and less than or equal to 50 nm, more preferably greater than orequal to 10 nm and less than or equal to 30 nm. The thickness of theinsulating film 131 can be greater than or equal to 30 nm and less thanor equal to 500 nm, preferably greater than or equal to 150 nm and lessthan or equal to 400 nm.

In the case where a nitride insulating film is used as the insulatingfilm 132, an insulating film having a barrier property against nitrogenis preferably used as one or both of the insulating film 129 and theinsulating film 131. For example, a dense oxide insulating film can havea barrier property against nitrogen. Specifically, an oxide insulatingfilm which can be etched at a rate less than or equal to 10 nm perminute when the temperature is 25° C. and 0.5 wt % of hydrofluoric acidis used is preferably used.

In the case where an oxide insulating film containing nitrogen, such asa silicon oxynitride film or a silicon nitride oxide film, is used asone or both of the insulating film 129 and the insulating film 131, thenitrogen concentration measured by secondary ion mass spectrometry(SIMS) is higher than or equal to the lower limit of measurement by SIMSand lower than 3×10²⁰ atoms/cm³, preferably higher than or equal to1×10¹⁸ atoms/cm³ and lower than or equal to 1×10²⁰ atoms/cm³. In thatcase, the amount of nitrogen which enters the multilayer film 111including the oxide semiconductor layer included in the transistor 103can be reduced and the number of defects in the nitrogen-containingoxide insulating film itself can be reduced.

As the insulating film 132, a nitride insulating film with a lowhydrogen content may be provided. The nitride insulating film is asfollows, for example: the number of hydrogen molecules released from thenitride insulating film is less than 5.0×10²¹ molecules/cm³, preferablyless than 3.0×10²¹ molecules/cm³, more preferably less than 1.0×10²¹molecules/cm³ when measured by TDS.

The insulating film 132 has a thickness large enough to prevent entry ofimpurities such as hydrogen and water from the outside. For example, thethickness can become greater than or equal to 50 nm and less than orequal to 200 nm, preferably greater than or equal to 50 nm and less thanor equal to 150 nm, and further preferably greater than or equal to 50nm and less than or equal to 100 nm.

The pixel electrode 121 is formed of a light-transmitting conductivematerial such as an In—Sn-based oxide, an In—W-based oxide, anIn—Zn—W-based oxide, an In—Ti-based oxide, an In—Ti—Sn-based oxide, anIn—Zn-based oxide, or an In—Si—Sn-based oxide.

Next, a structure of the element portion on the second substrate 150 isdescribed. The element portion includes a light-blocking film 152, anelectrode (a counter electrode 154) which is on the light-blocking film152 and faces the pixel electrode 121, and an insulating film 156 whichis on the counter electrode 154 and functions as an alignment film.

For the second substrate 150, a material similar to that used for thefirst substrate 102 can be used.

The light-blocking film 152 prevents the transistor 103 from beingirradiated with backlight or light from the outside. The light-blockingfilm 152 can be formed using a material such as a metal or an organicresin including a pigment and may be provided in a region outside thepixel portion 100, such as over the scan line driver circuit 104 andover the signal line driver circuit 106 (see FIGS. 1A to 1C), as well asover the transistor 103 in the pixel 101.

Note that a coloring film (also referred to as color filter) whichtransmits light with a predetermined wavelength may be provided betweenthe adjacent light-blocking films 152. In addition, an overcoat film maybe provided between the counter electrode 154, and the light-blockingfilm 152 and the coloring film in order to suppress dispersion ofimpurities from the light-blocking film 152, the coloring film, and thelike to the liquid crystal layer 160 side.

The counter electrode 154 is formed using any of the conductivematerials having light-transmitting properties which are given as thoseused for the pixel electrode 121 as appropriate.

The liquid crystal element 108 includes the pixel electrode 121, thecounter electrode 154, and a liquid crystal layer 160. The liquidcrystal layer 160 is sandwiched between the insulating film 158 which isprovided in the element portion over the first substrate 102 andfunctions as an alignment film and the insulating film 156 which isprovided in the element portion on the second substrate 150 andfunctions as an alignment film. Further, the pixel electrode 121overlaps with the counter electrode 154 with the liquid crystal layer160 interposed therebetween.

As described above, according to one embodiment of the presentinvention, the use of the multilayer film including the oxidesemiconductor layer for the transistor 103 and the capacitor 105 allowsa display device to be manufactured over a large substrate; thus, thedisplay device can be manufactured at low cost. In addition, since themultilayer film 119 used for the capacitor 105 has a light-transmittingproperty, the aperture ratio of a pixel can be increased, so that thedisplay device can have low power consumption. Moreover, since a channelis formed in a layer of the multilayer film 111 used for the transistor103, which is not in contact with the gate insulating film 127, thetransistor 103 can have stable electrical characteristics, so that thedisplay device can have high reliability.

Further, in the display device of one embodiment of the presentinvention, a region of the pixel 101 in which the light-blocking film152 is provided can be reduced or removed in such a manner thatpolarization axes of polarizing members (polarizing substrates) areprovided in parallel to set the display mode of the display device to anormally-black mode in which the liquid crystal element 108 does nottransmit light from a light source device such as a backlight with novoltage applied. As a result, the aperture ratio can be improved even inthe case where the size of one pixel is small as in a high-definitiondisplay device having a pixel density of 200 ppi or higher, andfurthermore 300 ppi or more. Further, the aperture ratio can be furtherimproved by using the light-transmitting capacitor 105.

Embodiment 2

In this embodiment, a method for manufacturing the element portionprovided over the first substrate 102 of the display device illustratedin FIGS. 3A to 3C in Embodiment 1 is described with reference to FIGS.4A and 4B and FIGS. 5A and 5B.

<Method for Manufacturing Display Device>

First, the scan line 107 and the capacitor line 115 are formed over thefirst substrate 102. An insulating film 126 which is to be processedinto the gate insulating film 127 later is formed so as to cover thescan line 107 and the capacitor line 115. The multilayer film 111 isformed over a portion of the insulating film 126 which overlaps with thescan line 107. The multilayer film 119 is formed so as to overlap with aregion where the pixel electrode 121 is to be formed later (see FIG.4A).

The scan line 107 and the capacitor line 115 can be formed in such amanner that a conductive film is formed using any of the materialsdescribed in Embodiment 1, a mask is formed over the conductive film,and the conductive film is processed using the mask. The conductive filmcan be formed by any of a variety of deposition methods such as anevaporation method, a PE-CVD method, a sputtering method, and a spincoating method. Note that there is no particular limitation on thethickness of the conductive film, and the thickness of the conductivefilm can be determined in consideration of time needed for theformation, desired resistivity, or the like. As the mask, for example, aresist mask formed through a photolithography process can be used. Theconductive film can be processed by one of or both dry etching and wetetching.

The insulating film 126 can be formed using a material which can be usedfor the gate insulating film 127, by any of a variety of depositionmethods such as a PE-CVD method and a sputtering method.

The multilayer film 111 and the multilayer film 119 can be formed usingany of the materials described in Embodiment 1. Note that the oxidefilms included in the multilayer film 111 and the multilayer film 119are preferably formed in succession in a vacuum. By forming the oxidefilms in succession in a vacuum, entry of impurities into the interfacebetween the oxide films can be suppressed.

The multilayer film 111 and the multilayer film 119 can be formed by asputtering method, a coating method, a pulsed laser deposition method, alaser ablation method, or the like. Alternatively, when a printingmethod is employed, the multilayer films 111 and 119 which are separatefrom each other can be formed directly on the insulating film 126.

In the case where the multilayer films 111 and 119 are formed by asputtering method, an RF power supply device, an AC power supply device,a DC power supply device, or the like can be used as appropriate as apower supply device for generating plasma. As a sputtering gas, a raregas (typically argon), an oxygen gas, or a mixed gas of a rare gas andoxygen is used as appropriate. In the case of using the mixed gas of arare gas and oxygen, the proportion of oxygen is preferably higher thanthat of a rare gas. Further, a target may be appropriately selected inaccordance with the composition of the oxide semiconductor film to beformed.

The multilayer films 111 and 119 can be processed by one or both of dryetching and wet etching. The etching conditions (e.g., an etching gas oran etching solution, etching time, and temperature) are set asappropriate depending on the material so that the multilayer films 111and 119 can be etched to have a desired shape.

Heat treatment is preferably performed after the multilayer films 111and 119 are formed so that the multilayer films 111 and 119 aresubjected to dehydrogenation or dehydration. The temperature of the heattreatment is typically higher than or equal to 150° C. and lower thanthe strain point of the substrate, preferably higher than or equal to200° C. and lower than or equal to 450° C., further preferably higherthan or equal to 300° C. and lower than or equal to 450° C. Note thatthe heat treatment may be performed on the multilayer film which has notbeen processed into the multilayer films 111 and 119.

A heat treatment apparatus used in the heat treatment is not limited toan electric furnace; as the heat treatment apparatus, an apparatus whichheats an object using thermal conduction or thermal radiation given by amedium such as a heated gas may be used. For example, an RTA (rapidthermal anneal) apparatus such as a GRTA (gas rapid thermal anneal)apparatus or an LRTA (lamp rapid thermal anneal) apparatus can be used.An LRTA apparatus is an apparatus for heating an object to be processedby radiation of light (an electromagnetic wave) emitted from a lamp suchas a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas.

The heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which the water content is 20 ppm or less,preferably 1 ppm or less, further preferably 10 ppb or less), or a raregas (argon, helium, or the like). The atmosphere of nitrogen, oxygen,ultra-dry air, or a rare gas preferably does not contain hydrogen,water, and the like. Alternatively, the heat treatment may be performedunder an atmosphere of an inert gas first, and then under an oxygenatmosphere. The treatment time is 3 minutes to 24 hours.

In the case where a base insulating film is provided between the firstsubstrate 102, and the scan line 107, the capacitor line 115, and thegate insulating film 127, the base insulating film can be formed usingany of the following: silicon oxide, silicon oxynitride, siliconnitride, silicon nitride oxide, gallium oxide, yttrium oxide, aluminumoxide, aluminum oxynitride, and the like. The use of silicon nitride,gallium oxide, yttrium oxide, aluminum oxide, or the like for the baseinsulating film leads to suppression of diffusion of impurities typifiedby an alkali metal, water, and hydrogen into the multilayer film 111from the first substrate 102. The base insulating film can be formed bya sputtering method or a PE-CVD method.

Next, the opening 123 reaching the capacitor line 115 is formed in theinsulating film 126 to form the gate insulating film 127. After that,the signal line 109 part of which functions as the source electrode ofthe transistor 103, the conductive film 113 part of which functions asthe drain electrode of the transistor 103, and the conductive film 125which electrically connects the multilayer film 119 and the capacitorline 115 are formed (see FIG. 4B).

The opening 123 can be formed in such a manner that a mask is formed soas to expose part of a portion of the insulating film 126 which overlapswith the capacitor line 115 and processing is performed using the mask.The formation of the mask and the processing can be performed in mannerssimilar to those of the scan line 107 and the capacitor line 115.

The signal line 109 and the conductive films 113 and 125 can be formedas follows: a conductive film is formed using a material which can beused for the signal line 109 and the conductive films 113 and 125, amask is formed over the conductive film, and processing is performedusing the mask. The formation of the mask and the processing can beperformed in manners similar to those of the scan line 107 and thecapacitor line 115.

Next, an insulating film 128 is formed over the multilayer films 111 and119, the signal line 109, the conductive films 113 and 125, and the gateinsulating film 127. An insulating film 130 is formed over theinsulating film 128. An insulating film 133 is formed over theinsulating film 130 (see FIG. 5A).

Note that it is preferable that the insulating films 128, 130, and 133be formed in succession in a vacuum. In such a case, entry of impuritiesinto each interface can be suppressed. Further, in FIG. 5A, theinterface between the insulating film 128 and the insulating film 130 isshown by a dotted line. In the case where the insulating film 128 andthe insulating film 130 are formed using the same kind of material, theinterface between the insulating film 128 and the insulating film 130cannot be clearly defined in some cases.

The insulating film 128 can be formed using a material that can be usedfor the insulating film 129, by any of a variety of deposition methodssuch as a PE-CVD method and a sputtering method. The insulating film 130can be formed using a material that can be used for the insulating film131. The insulating film 133 can be formed using a material that can beused for the insulating film 132.

The insulating film 128 (the insulating film 129) can be formed underthe following formation conditions. Here, as an oxide insulating film, asilicon oxide film or a silicon oxynitride film is formed. As for theformation conditions, the substrate placed in a treatment chamber of aPE-CVD apparatus, which is vacuum-evacuated, is held at a temperaturehigher than or equal to 180° C. and lower than or equal to 400° C.,preferably higher than or equal to 200° C. and lower than or equal to370° C., a deposition gas containing silicon and an oxidizing gas areintroduced as a source gas into the treatment chamber, the pressure inthe treatment chamber is greater than or equal to 20 Pa and less than orequal to 250 Pa, preferably greater than or equal to 40 Pa and less thanor equal to 200 Pa, and high-frequency power is supplied to an electrodeprovided in the treatment chamber.

Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, and silane fluoride. Examples of theoxidizing gas include oxygen, ozone, dinitrogen monoxide, and nitrogendioxide.

By setting the ratio of the amount of the oxidizing gas to the amount ofthe deposition gas containing silicon to 100 or higher, the hydrogencontent in the insulating film 128 (the insulating film 129) can bereduced and dangling bonds in the insulating film 128 (the insulatingfilm 129) can be reduced. Oxygen released from the insulating film 130(the insulating film 131) is captured by the dangling bonds in theinsulating film 128 (the insulating film 129) in some cases; thus, inthe case where the dangling bonds in the insulating film 128 (theinsulating film 129) are reduced, oxygen in the insulating film 130 (theinsulating film 131) can enter the multilayer films 111 and 119efficiently to fill oxygen vacancies in the multilayer films 111 and119. As a result, the amount of hydrogen entering the oxidesemiconductor film can be reduced and oxygen vacancies in the oxidesemiconductor film can be reduced.

In the case where the above oxide insulating film which includes anoxygen excess region or the above oxide insulating film in which theoxygen content is higher than that in the stoichiometric composition isused as the insulating film 130 (the insulating film 131), theinsulating film 130 (the insulating film 131) can be formed under thefollowing formation conditions. Here, as the oxide insulating film, asilicon oxide film or a silicon oxynitride film is formed. As for theformation conditions, the substrate placed in a treatment chamber of aPE-CVD apparatus, which is vacuum-evacuated, is held at a temperaturehigher than or equal to 180° C. and lower than or equal to 260° C.,preferably higher than or equal to 180° C. and lower than or equal to230° C., the pressure in the treatment chamber is greater than or equalto 100 Pa and less than or equal to 250 Pa, preferably greater than orequal to 100 Pa and less than or equal to 200 Pa with introduction of asource gas into the treatment chamber, and high-frequency power that ishigher than or equal to 0.17 W/cm² and lower than or equal to 0.5 W/cm²,preferably, higher than or equal to 0.25 W/cm² and lower than or equalto 0.35 W/cm² is supplied to an electrode provided in the treatmentchamber.

As the source gas of the insulating film 130 (the insulating film 131),a source gas which can be used for the insulating film 128 (theinsulating film 129) can be used.

As for the formation conditions of the insulating film 130, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the decompositionefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas proceeds; therefore, theoxygen content in the insulating film 130 is higher than that in thestoichiometric composition. However, in the case where the substratetemperature is within the above temperature range, the bond betweensilicon and oxygen is weak, and accordingly, part of oxygen is releasedby heating. Thus, it is possible to form an oxide insulating film inwhich the oxygen content is higher than that in the stoichiometriccomposition and from which part of oxygen is released by heating. Theinsulating film 128 is provided over the multilayer film 111.Accordingly, in the process for forming the insulating film 130, theinsulating film 128 serves as a protective film of the multilayer film111. Thus, even when the insulating film 130 is formed using thehigh-frequency power having a high power density, damage to themultilayer film 111 is not significant.

By increasing the thickness of the insulating film 130, a larger amountof oxygen is released by heating; thus, the insulating film 130 ispreferably formed thicker than the insulating film 128. Since theinsulating film 128 is provided, favorable coverage can be achieved evenwhen the insulating film 130 is formed thick.

In the case where a nitride insulating film with a low hydrogen contentis used as the insulating film 133, the insulating film 133 can beformed under the following formation conditions. Here, as the nitrideinsulating film, a silicon nitride film is formed. The substrate placedin a treatment chamber of the PE-CVD apparatus, which isvacuum-evacuated, is held at a temperature higher than or equal to 80°C. and lower than or equal to 400° C., preferably higher than or equalto 200° C. and lower than or equal to 370° C., the pressure is greaterthan or equal to 100 Pa and less than or equal to 250 Pa, preferablygreater than or equal to 100 Pa and less than or equal to 200 Pa withintroduction of a source gas into the treatment chamber, andhigh-frequency power is supplied to an electrode provided in thetreatment chamber.

As the source gas of the insulating film 133, a deposition gascontaining silicon, a nitrogen gas, and an ammonia gas are preferablyused. Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, and silane fluoride. Further, the flow rateof nitrogen is preferably 5 times to 50 times that of ammonia, furtherpreferably 10 times to 50 times that of ammonia. The use of ammonia asthe source gas facilitates decomposition of nitrogen and the depositiongas containing silicon. This is because ammonia is dissociated by plasmaenergy or heat energy, and energy generated by the dissociationcontributes to decomposition of a bond of the deposition gas moleculescontaining silicon and a bond of nitrogen molecules. Under the aboveconditions, a silicon nitride film which has a low hydrogen content andcan suppress entry of impurities such as hydrogen and water from theoutside can be formed.

It is preferable that heat treatment be performed at least afterformation of the insulating film 130 so that excess oxygen contained inthe insulating film 128 or the insulating film 130 enters the multilayerfilm 111 to fill oxygen vacancies in the multilayer film 111. The heattreatment can be appropriately performed according to the details ofheat treatment for dehydration or dehydrogenation of the multilayerfilms 111 and 119.

Then, the opening 117 reaching the conductive film 113 is formed inregions of the insulating film 128, the insulating film 130, and theinsulating film 133 which overlap with the conductive film 113. Byforming the opening 117, each of the insulating films 128, 130, and 133is divided, whereby the insulating film 129, the insulating film 131,and the insulating film 132 are formed. After that, a light-transmittingconductive film is formed over the conductive film 113, the insulatingfilm 129, the insulating film 131, and the insulating film 132 andunnecessary regions are removed, whereby the pixel electrode 121 isformed (see FIG. 5B).

The opening 117 can be formed in a manner similar to that of the opening123. The pixel electrode 121 can be formed in such a manner that alight-transmitting conductive film is formed using any of the materialsgiven above in contact with the conductive film 113 through the opening117, a mask is formed over the conductive film, and processing isperformed using the mask. The formation of the mask and the processingcan be performed in manners similar to those of the scan line 107 andthe capacitor line 115.

Through the above process, the display device which is one embodiment ofthe present invention can be manufactured.

As described above, according to one embodiment of the presentinvention, multilayer films each including an oxide semiconductor layerare used in the transistor 103 and the capacitor 105, whereby a displaydevice can be manufactured over a large substrate; thus, a displaydevice with low production cost can be provided. Further, the multilayerfilm 119 included in the capacitor 105 has a light-transmittingproperty; thus, the aperture ratio of the pixel becomes high, so that adisplay device with low power consumption can be provided. Further, achannel is formed in a layer, which is not in contact with the gateinsulating film 127, of the multilayer film 111 used for the transistor103, so that a transistor with stable electrical characteristics can beformed and a display device with high reliability can be manufactured.

Note that this embodiment may be combined as appropriate with astructure or the like described in the other embodiments.

Embodiment 3

In this embodiment, structures of display devices each of which is oneembodiment of the present invention, which are different from thestructure in Embodiment 1, are described with reference to FIG. 6, FIGS.7A to 7C, FIG. 8, FIGS. 9A to 9C, FIG. 10, and FIGS. 11A to 11C. Notethat in the display devices illustrated in FIG. 6, FIGS. 7A to 7C, FIG.8, FIGS. 9A to 9C, FIG. 10, and FIGS. 11A to 11C, a liquid crystallayer, elements formed on the second substrate on the opposite side, andthe like are not illustrated because the crystal layer, elements, andthe like are similar to those illustrated in FIGS. 3A to 3C.

Modification Example 1 of Structure of Display Device

First, a modification example 1 of a structure of a display device isdescribed with reference to FIG. 6 and FIGS. 7A to 7C. Here, only acapacitor 165 different from the capacitor 105 described with referenceto FIG. 2 and FIGS. 3A to 3C is described. FIG. 6 is a top view of apixel 161 and FIGS. 7A to 7C are cross-sectional views taken alongdashed-dotted line C1-C2 and dashed-dotted line D1-D2 in FIG. 6.

In the pixel 161, the conductive film 167 is in contact with themultilayer film 119 along the outer periphery thereof and is in contactwith the capacitor line 115 through the opening 123. The conductive film167 is formed in the same formation process as the signal line 109 partof which functions as the source electrode of the transistor 103 and theconductive film 113 part of which functions as the drain electrode ofthe transistor 103 and thus may have a light-blocking property; for thisreason, the area of the conductive film 167 as seen from above ispreferably small. However, the conductive film 167 may function as anauxiliary wiring, and a practitioner can determine the most suitableshape as appropriate. The structure of the pixel 161 in FIG. 6 issimilar to that in FIG. 2, except for the conductive film 167.

As illustrated in FIG. 7A, in the pixel 161, the conductive film 167 isprovided so as to cover end portions of the multilayer film 119 of thecapacitor 165.

FIG. 7B and FIG. 7C show an enlarged view of a region α (including thegate insulating film 127, the multilayer film 111, the signal line 109,and the insulating film 129) of the display device illustrated in FIG.7A and an enlarged view of a region β (including the gate insulatingfilm 127, the multilayer film 119, the conductive film 167, and theinsulating film 129) thereof, respectively.

In this embodiment, the structures of the multilayer film 111 and themultilayer film 119 are similar to the structures illustrated in FIG. 3Band FIG. 3C.

In the structure illustrated in FIG. 6 and FIGS. 7A to 7C, theconductive film 167 is formed into a loop shape; however, a portion ofthe conductive film 167, which is in contact with the multilayer film119, does not have to be entirely electrically connected to thecapacitor line 115. In other words, a conductive film formed in the sameformation process as the conductive film 167 may be provided in contactwith the multilayer film 119 so as to be separate from the conductivefilm 167.

With the structure illustrated in FIG. 6 and FIGS. 7A to 7C, the contactresistance between the multilayer film which is the other electrodeincluded in the capacitor and the conductive film which is electricallyconnected to the capacitor line can be reduced. Further, in the casewhere the conductivity of the other electrode included in the capacitoris low, the conductive film functions as an auxiliary electrode.

Modification Example 2 of Structure of Display Device

Next, a modification example 2 of a structure of a display device isdescribed with reference to FIG. 8 and FIGS. 9A to 9C. Here, a capacitor175 different from the capacitor 105 described with reference to FIG. 2and FIGS. 3A to 3C is described. FIG. 8 is a top view of a pixel 171 andFIGS. 9A to 9C are cross-sectional views taken along dashed-dotted lineE1-E2 and dashed-dotted line F1-F2 in FIG. 8.

In the pixel 171, an opening 139 reaching the multilayer film 119 isformed in the insulating film 129, the insulating film 131, and theinsulating film 132, and an opening 138 reaching a conductive film 135is formed in the gate insulating film 127, the insulating film 129, theinsulating film 131, and the insulating film 132. Further, a conductivefilm 137 is formed so as to cover the opening 139, the opening 138, andthe insulating film 132.

In the capacitor 175, a pixel electrode 124 functions as one electrodeand the multilayer film 119 functions as the other electrode. Note thatthe multilayer film 119 is connected to the conductive film 135 formedin the same process as the scan line 107 through the conductive film 137formed in the same process as the pixel electrode 124. By using such aconnection method, the openings 117, 139, and 138 can be formed in thesame process; thus, the number of masks can be reduced.

FIG. 9B and FIG. 9C show an enlarged view of a region α (including thegate insulating film 127, the multilayer film 111, the signal line 109,and the insulating film 129) of the display device illustrated in FIG.9A and an enlarged view of a region β (including the gate insulatingfilm 127, the multilayer film 119, and the insulating film 129) thereof,respectively.

In this embodiment, the structures of the multilayer film 111 and themultilayer film 119 are similar to the structures illustrated in FIG. 3Band FIG. 3C.

Modification Example 3 of Structure of Display Device

Next, a modification example 3 of a structure of a display device isdescribed with reference to FIG. 10 and FIGS. 11A to 11C. Here, acapacitor 185 different from the capacitor 175 described with referenceto FIG. 8 and FIGS. 9A to 9C is described. FIG. 10 is a top view of apixel 181 and FIGS. 11A to 11C are cross-sectional views taken alongdashed-dotted line G1-G2 and dashed-dotted line H1-H2 in FIG. 10.

In the pixel 181, an opening 149 reaching the conductive film 148 formedin the same process as the conductive film 113 is formed in theinsulating film 129, the insulating film 131, and the insulating film132, and the opening 138 reaching the conductive film 135 is formed inthe gate insulating film 127, the insulating film 129, the insulatingfilm 131, and the insulating film 132. Further, the conductive film 137is formed so as to cover the opening 138, the opening 149, and theinsulating film 132.

In the capacitor 185, the pixel electrode 124 functions as one electrodeand the multilayer film 119 functions as the other electrode. Note thatthe multilayer film 119 is connected to the conductive film 135 formedin the same process as the scan line 107 through the conductive film 148and the conductive film 137 which is formed in the same process as thepixel electrode 124. By using such a connection method, the openings117, 149, and 138 can be formed in the same process; thus, the number ofmasks can be reduced. Further, the connection resistance between themultilayer film 119 and the conductive film 137 can be reduced with theconductive film 148 provided therebetween.

FIG. 11B and FIG. 11C show an enlarged view of a region α (including thegate insulating film 127, the multilayer film 111, the signal line 109,and the insulating film 129) of the display device illustrated in FIG.11A and an enlarged view of a region β (including the gate insulatingfilm 127, the multilayer film 119, and the insulating film 129) thereof,respectively.

In this embodiment, the structures of the multilayer film 111 and themultilayer film 119 are similar to the structures illustrated in FIG. 3Band FIG. 3C.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

Embodiment 4

In this embodiment, one embodiment which can be applied to a multilayerfilm including an oxide semiconductor layer in the transistor and thecapacitor included in the display device described in the aboveembodiments is described.

At least one layer of the multilayer film including the oxidesemiconductor layer is preferably formed using any of an oxidesemiconductor such as an amorphous oxide semiconductor, in which acrystal part is not clearly observed, a single crystal oxidesemiconductor, a polycrystalline oxide semiconductor, and an oxidesemiconductor including a crystal part (a c-axis aligned crystallineoxide semiconductor: CAAC-OS).

The CAAC-OS is one of oxide semiconductor films including a plurality ofcrystal parts, and most of the crystal parts each fit inside a cubewhose one side is less than 100 nm Thus, there is a case where a crystalpart included in the CAAC-OS fits inside a cube whose one side is lessthan 10 nm, less than 5 nm, or less than 3 nm. The density of defectstates of the CAAC-OS is lower than that of the microcrystalline oxidesemiconductor film. The CAAC-OS is described in detail below.

In a transmission electron microscope (TEM) image of the CAAC-OS, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS, a reduction in electron mobility due tothe grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS is formed (hereinafter, a surface over which the CAAC-OS isformed is referred to as a formation surface) or a top surface of theCAAC-OS, and is arranged in parallel to the formation surface or the topsurface of the CAAC-OS.

On the other hand, according to the TEM image of the CAAC-OS observed ina direction substantially perpendicular to the sample surface (plan TEMimage), metal atoms are arranged in a triangular or hexagonalconfiguration in the crystal parts. However, there is no regularity ofarrangement of metal atoms between different crystal parts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS.

A CAAC-OS is subjected to structural analysis with an X-ray diffraction(XRD) apparatus. For example, when the CAAC-OS including an InGaZnO₄crystal is analyzed by an out-of-plane method, a peak appears frequentlywhen the diffraction angle (2θ) is around 31°. This peak is derived fromthe (009) plane of the InGaZnO₄ crystal, which indicates that crystalsin the CAAC-OS have c-axis alignment, and that the c-axes are aligned ina direction substantially perpendicular to the formation surface or thetop surface of the CAAC-OS.

On the other hand, when the CAAC-OS is analyzed by an in-plane method inwhich an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS having c-axis alignment,while the directions of a-axes and b-axes are different between crystalparts, the c-axes are aligned in a direction parallel to a normal vectorof a formation surface or a normal vector of a top surface. Thus, eachmetal atom layer arranged in a layered manner observed in thecross-sectional TEM image corresponds to a plane parallel to the a-bplane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where a shape ofthe CAAC-OS is changed by etching or the like, the c-axis might not benecessarily parallel to a normal vector of a formation surface or anormal vector of a top surface of the CAAC-OS.

Further, the degree of crystallinity in the CAAC-OS is not necessarilyuniform. For example, in the case where crystal growth leading to theCAAC-OS occurs from the vicinity of the top surface of the film, thedegree of the crystallinity in the vicinity of the top surface is higherthan that in the vicinity of the formation surface in some cases.Further, when an impurity is added to the CAAC-OS, the crystallinity ina region to which the impurity is added is changed, and the degree ofcrystallinity in the CAAC-OS varies depending on regions.

Note that when the CAAC-OS with an InGaZnO₄ crystal is analyzed by anout-of-plane method, a peak of 2θ may also be observed at around 36°, inaddition to the peak of 2θ at around 31°. The peak of 2θ at around 36°indicates that a crystal having no c-axis alignment is included in partof the CAAC-OS. It is preferable that in the CAAC-OS, a peak of 2θappear at around 31° and a peak of 2θ do not appear at around 36°.

There are three methods for forming a CAAC-OS.

The first method is to form an oxide semiconductor film at a temperaturehigher than or equal to 100° C. and lower than or equal to 450° C. toform, in the oxide semiconductor film, crystal parts in which the c-axesare aligned in the direction parallel to a normal vector of a surfacewhere the oxide semiconductor film is formed or a normal vector of asurface of the oxide semiconductor film.

The second method is to form an oxide semiconductor film with a smallthickness and then heat it at a temperature higher than or equal to 200°C. and lower than or equal to 700° C. to form, in the oxidesemiconductor film, crystal parts in which the c-axes are aligned in thedirection parallel to a normal vector of a surface where the oxidesemiconductor film is formed or a normal vector of a surface of theoxide semiconductor film.

The third method is to form a first oxide semiconductor film with asmall thickness, then heat it at a temperature higher than or equal to200° C. and lower than or equal to 700° C., and form a second oxidesemiconductor film to form, in the second oxide semiconductor film,crystal parts in which the c-axes are aligned in the direction parallelto a normal vector of a surface where the second oxide semiconductorfilm is formed or to a normal vector of a surface of the second oxidesemiconductor film.

In a transistor using the CAAC-OS for an oxide semiconductor film,change in the electrical characteristics of the transistor due toirradiation with visible light or ultraviolet light is small. Thus, thetransistor using the CAAC-OS as the oxide semiconductor film has highreliability.

Further, it is preferable that the CAAC-OS be formed by a sputteringmethod using a polycrystalline oxide semiconductor target. When ionscollide with the target, a crystal region included in the target may beseparated from the target along an a-b plane; in other words, asputtered particle having a plane parallel to an a-b plane(flat-plate-like sputtered particle or pellet-like sputtered particle)may flake off from the target. In that case, the flat-plate-like orpellet-like sputtered particle reaches a surface where the CAAC-OS isformed while maintaining their crystal state, whereby the CAAC-OS can beformed.

The conditions described below are preferably employed for the formationof the CAAC-OS.

By reducing the amount of impurities entering the CAAC-OS during thedeposition, the crystal state can be prevented from being broken by theimpurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

By increasing the heating temperature of the surface where the CAAC-OSis formed (for example, the substrate heating temperature) during thedeposition, migration of a sputtered particle is likely to occur afterthe sputtered particle reaches the surface where the CAAC-OS is formed.Specifically, the temperature of the surface where the CAAC-OS is formedduring the deposition is higher than or equal to 100° C. and lower thanor equal to 740° C., preferably higher than or equal to 150° C. andlower than or equal to 500° C. By increasing the temperature of thesurface where the CAAC-OS is formed during the deposition, when theflat-plate-like or pellet-like sputtered particle reaches the surfacewhere the CAAC-OS is formed, migration occurs on the surface, so that aflat plane of the sputtered particle is attached to the surface.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is higher than or equal to 30 vol %, preferably 100 vol%.

As an example of the target, an In—Ga—Zn-based oxide target is describedbelow.

The In—Ga—Zn oxide target, which is polycrystalline, is made by mixingInO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in a predeterminedmolar ratio, applying pressure, and performing heat treatment at atemperature higher than or equal to 1000° C. and lower than or equal to1500° C. This pressure treatment may be performed while cooling isperformed or may be performed while heating is performed. Note that X,Y, and Z are each a given positive number. Here, the predetermined molarratio of InO_(X) powder to GaO_(Y) powder and ZnO_(Z) powder is, forexample, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds ofpowder and the molar ratio for mixing powder may be determined asappropriate depending on the desired target.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and the likedescribed in the other embodiments.

Embodiment 5

In this embodiment, an example in which a system-on-panel is formed byforming the display device an example of which is described in the aboveembodiments and part or all of a driver circuit over a substrate where apixel portion is formed is described with reference to FIGS. 12A to 12C,FIGS. 13A and 13B, and FIGS. 14A to 14C. FIGS. 13A and 13B are each across-sectional view illustrating a cross-sectional portion taken alongthe dashed-dotted line M-N in FIG. 12B. Note that in FIGS. 13A and 13B,only part of the structure of the pixel portion is illustrated.

In FIG. 12A, a sealant 905 is provided so as to surround a pixel portion902 provided over a first substrate 901, and the pixel portion 902 issealed with a second substrate 906. In FIG. 12A, a signal line drivercircuit 903 and a scan line driver circuit 904 each are formed using asingle crystal semiconductor or a polycrystalline semiconductor over asubstrate prepared separately, and mounted in a region different fromthe region surrounded by the sealant 905 over the first substrate 901.Further, various signals and potentials are supplied to the signal linedriver circuit 903, the scan line driver circuit 904, and the pixelportion 902 from flexible printed circuits (FPCs) 918 a and 918 b.

In FIGS. 12B and 12C, the sealant 905 is provided so as to surround thepixel portion 902 and the scan line driver circuit 904 which areprovided over the first substrate 901. The second substrate 906 isprovided over the pixel portion 902 and the scan line driver circuit904. Thus, the pixel portion 902 and the scan line driver circuit 904are sealed together with a display element by the first substrate 901,the sealant 905, and the second substrate 906. In FIGS. 12B and 12C, asignal line driver circuit 903 which is formed using a single crystalsemiconductor or a polycrystalline semiconductor over a substrateprepared separately is mounted in a region different from the regionsurrounded by the sealant 905 over the first substrate 901. In FIGS. 12Band 12C, various signals and potentials are supplied to the signal linedriver circuit 903, the scan line driver circuit 904, and the pixelportion 902 from an FPC 918.

Although FIGS. 12B and 12C each illustrate an example in which thesignal line driver circuit 903 is formed separately and mounted on thefirst substrate 901, one embodiment of the present invention is notlimited to this structure. The scan line driver circuit may beseparately formed and then mounted, or only part of the signal linedriver circuit or part of the scan line driver circuit may be separatelyformed and then mounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method, or the like canbe used. FIG. 12A illustrates an example in which the signal line drivercircuit 903 and the scan line driver circuit 904 are mounted by a COGmethod. FIG. 12B illustrates an example in which the signal line drivercircuit 903 is mounted by a COG method. FIG. 12C illustrates an examplein which the signal line driver circuit 903 is mounted by a TAB method.

The display device includes, in its category, a panel in which a displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel.

A display device in this specification refers to an image displaydevice, a display device, or a light source (including a lightingdevice). Furthermore, the display device also includes the followingmodules in its category: a module to which a connector such as an FPC,or a TCP is attached; a module having a TCP at the tip of which aprinted wiring board is provided; and a module in which an integratedcircuit (IC) is directly mounted on a display element by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors and any of thetransistors which are described in the above embodiments can be used.

As the display element provided in the display device, a liquid crystalelement (also referred to as liquid crystal display element) or alight-emitting element (also referred to as light-emitting displayelement) can be used. A light-emitting element includes, in its scope,an element whose luminance is controlled by current or voltage, andspecifically includes an inorganic electroluminescent (EL) element, anorganic EL element, and the like. Furthermore, a display medium whosecontrast is changed by an electric effect, such as electronic ink, canbe used. FIGS. 13A and 13B illustrate an example of a liquid crystaldisplay device using a liquid crystal element as a display element.

The liquid crystal display device illustrated in FIG. 13A is a verticalelectric field mode liquid crystal display device. The liquid crystaldisplay device includes a connection terminal electrode 915 and aterminal electrode 916. The connection terminal electrode 915 and theterminal electrode 916 are electrically connected to a terminal includedin the FPC 918 through an anisotropic conductive agent 919.

The connection terminal electrode 915 is formed using the sameconductive film as a first electrode 930. The terminal electrode 916 isformed using the same conductive film as source and drain electrodes oftransistors 910 and 911.

Further, the pixel portion 902 and the scan line driver circuit 904which are provided over the first substrate 901 each include a pluralityof transistors, and the transistor 910 included in the pixel portion 902and the transistor 911 included in the scan line driver circuit 904 areillustrated as an examples. An insulating film 924 corresponding to theinsulating films 129, 131, and 132 described in Embodiment 1 is providedover the transistors 910 and 911. Note that an insulating film 923 is aninsulating film serving as a base film.

In this embodiment, the transistor described in the above embodimentscan be applied to the transistors 910 and 911. A capacitor 926 is formedusing an oxide semiconductor film 927, the insulating film 924, and thefirst electrode 930. The oxide semiconductor film 927 is connected to acapacitor wiring 929 through an electrode 928. The electrode 928 isformed using the same conductive film as the source and drain electrodesof each of the transistors 910 and 911. The capacitor wiring 929 isformed using the same conductive film as a gate electrode of each of thetransistors 910 and 911. Although the capacitor described in Embodiment1 is illustrated as the capacitor 926 here, any of the capacitors in theother embodiments may be used as appropriate.

Moreover, an example in which a conductive film 917 is provided over theinsulating film 924 so as to overlap with a channel formation region ofthe oxide semiconductor film of the transistor 911 included in the scanline driver circuit is illustrated. In this embodiment, the conductivefilm 917 is formed using the same conductive film as the first electrode930 and a first electrode 940. By providing the conductive film 917 soas to overlap with a channel formation region of the oxide semiconductorfilm, the amount of change in the threshold voltage of the transistor911 between before and after a reliability test (e.g., a biastemperature (BT) stress test) can be further reduced. The conductivefilm 917 may have the same potential as or a potential different fromthat of the gate electrode of the transistor 911, and the conductivefilm 917 can serve as a second gate electrode. The potential of theconductive film 917 is set to a ground potential, a source potential, afixed potential, or a potential of a gate electrode, for example.

In addition, the conductive film 917 has a function of blocking anexternal electric field. In other words, the conductive film 917 has afunction of preventing an external electric field (particularly, afunction of preventing static electricity) from affecting the inside (acircuit portion including the transistor). Such a blocking function ofthe conductive film 917 can prevent variation in the electriccharacteristics of the transistor due to an influence of an externalelectric field such as static electricity. Further, the thresholdvoltage of the transistor can be controlled. Note that although thetransistors included in the scan line driver circuit are illustrated inFIGS. 13A and 13B, in a manner similar to that of the transistor 911, atransistor included in the signal line driver circuit may have astructure in which a conductive film is provided over the insulatingfilm 924 so as to overlap with a channel formation region of the oxidesemiconductor film.

The transistor 910 provided in the pixel portion 902 is electricallyconnected to the display element to form a display panel. There is noparticular limitation on the kind of the display element as long asdisplay can be performed, and any of various kinds of display elementscan be employed.

A liquid crystal element 913 which is a display element includes thefirst electrode 930, a second electrode 931, and a liquid crystal layer908. An insulating film 932 and an insulating film 933 which serve asalignment films are provided so that the liquid crystal layer 908 isinterposed therebetween. The second electrode 931 is provided on thesecond substrate 906 side. The second electrode 931 overlaps with thefirst electrode 930 with the liquid crystal layer 908 interposedtherebetween.

The first electrode and the second electrode (each of which is alsoreferred to as a pixel electrode, a common electrode, a counterelectrode, or the like) for applying voltage to the display element mayhave light-transmitting properties or light-reflecting properties, whichdepends on the direction in which light is extracted, the position wherethe electrode is provided, and the pattern structure of the electrode.

The first electrode 930 and the second electrode 931 can be formed usingmaterials similar to those of the pixel electrode 121 and the counterelectrode 154 in Embodiment 1 as appropriate.

A spacer 935 is a columnar spacer obtained by selectively etching aninsulating film and is provided in order to control the distance (cellgap) between the first electrode 930 and the second electrode 931.Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, thermotropic liquid crystal, low-molecular liquid crystal,high-molecular liquid crystal, polymer-dispersed liquid crystal,ferroelectric liquid crystal, anti-ferroelectric liquid crystal, or thelike can be used. Such a liquid crystal material exhibits a cholestericphase, a smectic phase, a cubic phase, a chiral nematic phase, anisotropic phase, or the like depending on conditions.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase appears only in a narrowtemperature range, a liquid crystal composition in which a chiralmaterial is mixed is used for the liquid crystal layer in order toimprove the temperature range. Note that the alignment film is formedusing an organic resin containing hydrogen, water, or the like, whichmight degrade the electrical characteristics of the transistor in thedisplay device of one embodiment of the present invention. In view ofthe above, the use of liquid crystal which exhibits a blue phase for theliquid crystal layer 160 enables manufacture of the display device ofone embodiment of the present invention without an organic resin, sothat the display device can be highly reliable.

The first substrate 901 and the second substrate 906 are fixed in placeby a sealant 925. As the sealant 925, an organic resin such as athermosetting resin or a photocurable resin can be used. The sealant 925is in contact with the insulating film 924. Note that the sealant 925corresponds to the sealant 905 illustrated in FIGS. 12A to 12C.

In the liquid crystal display device, a black matrix (light-blockingfilm), an optical member (an optical substrate) such as a polarizingmember, a retardation member, or an anti-reflection member, and the likeare provided as appropriate. For example, circular polarization may beemployed by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

Since the transistor is easily broken due to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protection circuit is preferably formed using anonlinear element.

Next, a transverse electric field mode liquid crystal display device isdescribed with reference to FIG. 13B. FIG. 13B illustrates a liquidcrystal display device of a fringe field switching (FFS) mode, which isone of transverse electric field modes. A structure which is differentfrom that of the liquid crystal display device of the vertical electricfield mode illustrated in FIG. 13A is described.

In the liquid crystal display device illustrated in FIG. 13B, theconnection terminal electrode 915 is formed using the same conductivefilm as a first electrode 940, and the terminal electrode 916 is formedusing the same conductive film as the source and drain electrodes ofeach of the transistors 910 and 911.

A liquid crystal element 943 includes the first electrode 940 over theinsulating film 924, a second electrode 941, and the liquid crystallayer 908. Note that the liquid crystal element 943 can have a structuresimilar to that of the capacitor 105 described in Embodiment 1. Thefirst electrode 940 can be formed using, as appropriate, the materialfor the first electrode 930 illustrated in FIG. 13A. The planar shape ofthe first electrode 940 is a comb-like shape, a staircase-like shape, aladder-like shape, or the like. The second electrode 941 functions as acommon electrode and can be formed in a manner similar to that of themultilayer film 119 described in Embodiment 1. The insulating film 924is provided between the first electrode 940 and the second electrode941.

The second electrode 941 is connected to a common wiring 946 through anelectrode 945. The electrode 945 is formed using the same conductivefilm as the source and drain electrodes of each of the transistors 910and 911. The common wiring 946 is formed using the same conductive filmas the gate electrode of each of the transistors 910 and 911. Althoughthe description is made using the capacitor described in Embodiment 1 asthe liquid crystal element 943 here, any of the capacitors described inthe other embodiments can be used as appropriate.

FIGS. 14A to 14C illustrate an example of the liquid crystal displaydevice illustrated in FIG. 13A in which a common connection portion (padportion) for being electrically connected to the second electrode 931provided on the second substrate 906 is formed over the first substrate901.

The common connection portion is provided in a position which overlapswith the sealant for bonding the first substrate 901 and the secondsubstrate 906, and is electrically connected to the second electrode 931through conductive particles contained in the sealant. Alternatively,the common connection portion is provided in a position which does notoverlap with the sealant (except for the pixel portion) and a pastecontaining conductive particles is provided separately from the sealantso as to overlap with the common connection portion, whereby the commonconnection portion is electrically connected to the second electrode931.

FIG. 14A is a cross-sectional view of the common connection portiontaken along the line I-J in a top view of FIG. 14B.

A common potential line 975 is provided over the gate insulating film922 and is formed using the same material and through the same steps asa source electrode 971 and a drain electrode 973 of the transistor 910illustrated in FIGS. 14A to 14C.

Further, the common potential line 975 is covered with the insulatingfilm 924, and a plurality of openings are formed in the insulating film924 at positions overlapping with the common potential line 975. Theseopenings are formed through the same steps as a contact hole whichconnects the first electrode 930 and one of the source electrode 971 andthe drain electrode 973 of the transistor 910.

Further, the common potential line 975 is connected to the commonelectrode 977 through the openings. The common electrode 977 is providedover the interlayer insulating film 924 and formed using the samematerial and through the same steps as the connection terminal electrode915 and the first electrode 930 in the pixel portion.

In this manner, the common connection portion can be manufactured in thesame process as the switching element in the pixel portion 902.

The common electrode 977 is an electrode in contact with the conductiveparticles contained in the sealant, and is electrically connected to thesecond electrode 931 of the second substrate 906.

As illustrated in FIG. 14C, a common potential line 985 may be formedusing the same material and through the same steps as the gate electrodeof the transistor 910.

In the common connection portion in FIG. 14C, the common potential line985 is provided under the gate insulating film 922 and the insulatingfilm 924, and a plurality of openings are formed in the gate insulatingfilm 922 and the insulating film 924 at positions overlapping with thecommon potential line 985. These openings are formed by etching theinsulating film 924 and further selectively etching the gate insulatingfilm 922, through the same steps as a contact hole which connects thefirst electrode 930 and one of the source electrode 971 and the drainelectrode 973 of the transistor 910.

Further, the common potential line 985 is connected to the commonelectrode 987 through the openings. The common electrode 987 is providedover the insulating film 924 and formed using the same material andthrough the same steps as the connection terminal electrode 915 and thefirst electrode 930 in the pixel portion.

As described above, according to one embodiment of the presentinvention, the use of a multilayer film including an oxide semiconductorlayer for a transistor and a capacitor allows a display device to bemanufactured over a large substrate; thus, the display device can bemanufactured at low cost. In addition, since the multilayer film usedfor the capacitor has a light-transmitting property, the aperture ratioof a pixel can be increased, so that the display device can have lowpower consumption. Moreover, since a channel is formed in the layer inthe multilayer film used for the transistor which is not in contact witha gate insulating film, the transistor can have stable electricalcharacteristics, so that the display device can have high reliability.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and the likedescribed in the other embodiments.

Embodiment 6

The display device which is one embodiment of the present invention canbe applied to a variety of electronic appliances (including gamemachines). Examples of electronic appliances include a television device(also referred to as television or television receiver), a monitor of acomputer or the like, a camera such as a digital camera or a digitalvideo camera, a digital photo frame, a mobile phone, a portable gamemachine, a portable information terminal, an audio reproducing device, agame machine (e.g., a pachinko machine or a slot machine), and a gameconsole. Examples of these electronic appliances are illustrated inFIGS. 15A to 15C.

FIG. 15A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. The housing 9001 issupported by four leg portions 9002. Further, a power cord 9005 forsupplying power is provided for the housing 9001.

The display device described in any of the above embodiments can be usedfor the display portion 9003. Thus, the display portion 9003 can havehigh display quality.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her fingers or the like, the user can carryout operation of the screen and input of information. Further, when thetable may be made to communicate with home appliances or control thehome appliances, the display portion 9003 may function as a controldevice which controls the home appliances by operation on the screen.For example, with the use of the display device having an image sensorfunction, the display portion 9003 can have a touch-input function.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table 9000 can also be used as a television device. When atelevision device having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be efficiently used.

FIG. 15B illustrates a television device 9100. In the television device9100, a display portion 9103 is incorporated in a housing 9101 and animage can be displayed on the display portion 9103. Note that thehousing 9101 is supported by a stand 9105 here.

The television device 9100 can be operated with an operation switch ofthe housing 9101 or a separate remote controller 9110. Channels andvolume can be controlled with an operation key 9109 of the remotecontroller 9110 so that an image displayed on the display portion 9103can be controlled. Furthermore, the remote controller 9110 may beprovided with a display portion 9107 for displaying data output from theremote controller 9110.

The television device 9100 illustrated in FIG. 15B is provided with areceiver, a modem, and the like. With the receiver, general televisionbroadcasts can be received in the television device 9100. Further, whenthe television device 9100 is connected to a communication network bywired or wireless connection via the modem, one-way (from a transmitterto a receiver) or two-way (between a transmitter and a receiver orbetween receivers) data communication can be performed.

Any of the display devices described in the above embodiments can beused for the display portions 9103 and 9107. Thus, the television devicecan have high display quality.

FIG. 15C illustrates a computer 9200, which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like.

Any of the display devices described in the above embodiments can beused for the display portion 9203. Thus, the computer can have highdisplay quality.

The display portion 9203 has a touch-input function. When a user touchesa keyboard 9204 which are displayed on the display portion 9203 of thecomputer 9200 with his/her fingers or the like, the user can carry outoperation of the screen and input of information. Further, when thetable may be made to communicate with home appliances or control thehome appliances, the display portion 9203 may function as a controldevice which controls the home appliances by operation on the screen.

FIGS. 16A and 16B illustrate a foldable tablet terminal. In FIG. 16A,the tablet terminal is opened and includes a housing 9630, a displayportion 9631 a, a display portion 9631 b, a display-mode switchingbutton 9034, a power button 9035, a power-saving-mode switching button9036, a clip 9033, and an operation button 9038.

Any of the display devices described in the above embodiments can beused for the display portion 9631 a and the display portion 9631 b.Thus, the display quality of the tablet terminal can be improved.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9638 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region also has a touchpanel function is illustrated as an example, the structure of thedisplay portion 9631 a is not limited thereto. The whole area of thedisplay portion 9631 a may have a touch screen function. For example,the whole area of the display portion 9631 a can display keyboardbuttons and serve as a touch screen while the display portion 9631 b canbe used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch screen region 9632 b. When a keyboard display switchingbutton 9639 displayed on the touch panel is touched with a finger, astylus, or the like, a keyboard can be displayed on the display portion9631 b.

Touch input can be performed concurrently on the touch screen regions9632 a and 9632 b.

The display-mode switching button 9034 can switch display orientation(e.g., between landscape mode and portrait mode) and select a displaymode (switch between monochrome display and color display), for example.The power-saving-mode switching button 9036 can control displayluminance in accordance with the amount of external light in use of thetablet terminal detected by an optical sensor incorporated in thetablet. The tablet terminal may include another detection device such asa sensor for detecting orientation (e.g., a gyroscope or an accelerationsensor) in addition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 16A, one embodiment of the presentinvention is not limited to this example. The display portion 9631 a andthe display portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

In FIG. 16B, the tablet terminal is folded and includes the housing9630, a solar cell 9633, and a charge and discharge control circuit9634. Note that in FIG. 16B, an example in which the charge anddischarge control circuit 9634 includes the battery 9635 and the DCDCconverter 9636 is illustrated.

Since the tablet can be folded in two, the housing 9630 can be closedwhen not in use. Thus, the display portions 9631 a and 9631 b can beprotected, thereby providing a tablet with high endurance and highreliability for long-term use.

In addition, the tablet terminal illustrated in FIGS. 16A and 16B canhave a function of displaying various kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch screen, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630, so that thebattery 9635 can be charged efficiently. The use of a lithium ionbattery as the battery 9635 is advantageous in downsizing or the like.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 16B are described with reference to a blockdiagram of FIG. 16C. The solar cell 9633, the battery 9635, the DCDCconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are shown in FIG. 16C, and the battery 9635, the DCDCconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 in FIG. 16B.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofpower generated by the solar battery is raised or lowered by the DCDCconverter 9636 so that the power has a voltage for charging the battery9635. Then, when the power from the solar cell 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9637 soas to be a voltage needed for the display portion 9631. In addition,when display on the display portion 9631 is not performed, the switchSW1 is turned off and a switch SW2 is turned on so that charge of thebattery 9635 may be performed.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, without limitation thereon, the battery 9635may be charged using another power generation means such as apiezoelectric element or a thermoelectric conversion element (Peltierelement). For example, the battery 9635 may be charged with anon-contact power transmission module that transmits and receives powerwirelessly (without contact) to charge the battery or with a combinationof other charging means.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and the likedescribed in the other embodiments.

This application is based on Japanese Patent Application serial no.2012-202123 filed with Japan Patent Office on Sep. 13, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a transistorcomprising: a gate electrode; a first multilayer film comprising anoxide semiconductor layer comprising a channel formation region of thetransistor; and a gate insulating film between the gate electrode andthe first multilayer film; a pixel electrode electrically connected tothe transistor; and a capacitor comprising a second multilayer filmoverlapping with the pixel electrode, wherein the first multilayer filmcomprises the same layer structure as the second multilayer film, andwherein the oxide semiconductor layer is provided apart from the gateinsulating film.